Dozens of cases of children who died of abuse and neglect in Massachusetts over the last six years remain unresolved, the alleged abusers unpunished, because state investigations have been hamstrung by long delays in determining whether deaths were accidental or criminal, and the frequent inability to determine exactly how children died, the New England Center for Investigative Reporting has found.

The state medical examiner’s office, long under fire for delays in performing adult autopsies, is even slower when children die, taking an average of 242 days to find an official cause of death in abuse and neglect cases. Official death reports based on those findings sometimes take more than three years to complete, the Center found in its review of 102 cases, including the case of a one-month old boy who died in 2012 whose death determination is still pending.

The unresolved cases include several open homicide investigations, but also many others in which the medical examiner was unable to confirm whether wrongdoing was at play. In more than 40 percent of the cases reviewed, the medical examiner could not determine if the child’s death was an accident or a deliberate act.

As a series of high-profile child abuse deaths in Massachusetts make headlines – including the recent arraignment of the alleged killer of 2-year-old Bella Bond whose body was found in a trash bag on Deer Island – family members of children whose abuse drew less public notice question if justice will ever be served in their cases.

“We have been trying so hard to find out what happened. This has been going on for two years,’’ said Sharon Crawford of Whitinsville, who says her daughter called the medical examiner repeatedly, sometimes several times a day, for news about the 2013 case of her 10-year-old son, Isaiah Buckner, whose death had been linked by social workers to abuse and neglect.

But when the medical examiner’s report finally came Oct. 21, the cause of death was listed as “undetermined,” leaving criminal prosecutors little to go on.

“I just want justice for my grandson,” said Crawford.

State officials acknowledge that the medical examiner’s office has been plagued by delays, and is currently facing a backlog of 1,922 pending autopsy reports from 2011 to 2014. Daniel Bennett, secretary of the Executive Office of Public Safety and Security, said the office is trying to hire more examiners and other staff to reduce the backlog.

“There are failures within the system,’’ he said. The state, for example, did not have access to a doctor who could examine infant hearts for almost a year, an issue that was resolved in May. “The medical examiner’s office has been climbing out of a hole for the last two years,” he said.

Still, even with sufficient resources, the process is necessarily painstaking and slow in some cases, Bennett said. Infant deaths, for instance, sometimes require multiple scientific tests and the review of reports from law enforcement, the state Department of Children and Families and hospitals.

And not all cases can be resolved. Worcester District Attorney Joseph D. Early Jr. said that investigating and prosecuting child maltreatment deaths can be particularly challenging for law enforcement.

When young children die, there are often few clues about the cause. Unlike adults or older children, they generally leave no trail of evidence such as text messages, no a wide circle of adults who might have noticed or suspected signs of abuse, and little verbal capacity to tell anyone about their plight before death. Children also are often in the care of more than one person in the period leading up to their deaths, making it harder to identify a suspect.

“It tears your heart out in some of these cases,” Early said. “We can only go where the facts and the evidence take us.”

Beyond the heartbreak for families, the delays and uncertainty in these cases may also be putting other children at risk, say child advocates. Stephen C. Boos, medical director for the Family Advocacy Center at Baystate Children’s Hospital in Springfield said long delays can hurt investigations, potentially allowing killers to escape responsibility.

“It is scary to have potential child murderers running around with other children,” said Boos.

That very thought has haunted Shelly Medeiros for years.

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After 8-month-old Jay Hudson Bassett was rushed to a Worcester hospital on Thanksgiving, 2012, Medeiros, his maternal grandmother, quickly fixated on the bruise above his left eye. As the once-playful boy lay unconscious, Medeiros began a mantra of grief that hasn’t stopped to this day: “Something isn’t right.”

Police records show that UMass Memorial Medical Center hospital initially linked Jay’s death to Sudden Infant Death Syndrome – but Medeiros hounded authorities to look more closely. Eighteen months later, in May 2014, her worst fear was confirmed: The Office of the Chief Medical Examiner ruled Jay’s death a homicide caused by “blunt trauma” to the head and neck, inflicted by others.

Now, three years after Jay’s death, no one has been charged, and the investigation remains open. Jay’s parents were with the baby when he stopped breathing, police records show. They cut ties with Medeiros shortly after the child’s death and moved out of state. The couple, who have a new baby boy, declined to answer questions about Jay when a reporter visited last spring or left several phone calls and text messages over the last six months.

While there is no evidence that Jay’s parents are suspects in the open police investigation, Medeiros believes that, at the very least, they know what happened. And she believes the long delay in making a death ruling seriously undermined the investigation.

“In 18 months, everything changed,” said Medeiros, 51, of North Attleboro, who says her obsession with her grandson’s death keeps her awake at night. “Evidence wasn’t sealed off. My daughter and her boyfriend were allowed to leave town.”

The languishing child death investigations and cold cases are symptomatic of a state government that has often given low priority to abused and neglected youths, many specialists say. Recent highly publicized child abuse cases have spurred Gov. Charles Baker to propose reforms to help keep at-risk children safe. But much less has been said about what happens after children die. The New England Center analyzed all child maltreatment deaths reported by DCF from 2009 to 2013 for which a death certificate was available and found a discouraging pattern:

• The medical examiner’s office determined the cause and manner of death in only about one third of child maltreatment deaths within 90 days, a performance dramatically below the minimum standards set by the National Association of Medical Examiners, which expects 90 percent of autopsy reports to be done in that time. A top official at the National District Attorneys Association called the Massachusetts delays “unacceptable.”

• Some of the open criminal investigations into children who died of abuse and neglect between 2009 and 2013 are more than five years old, raising concerns among child advocates that they may have fallen through the cracks. In all, law enforcement agencies have open investigations into the deaths of at least 14 children on DCF’s list of abuse and neglect victims between 2009 and 2013.

• In 45 of the 102 child maltreatment fatalities reviewed by NECIR, medical examiners could not determine the manner of death – meaning they couldn’t decide between two or more causes, including whether a child died of an accident, homicide, suicide or natural events. The majority involve what DCF calls “unsafe sleep” deaths linked to parents who shared a bed or put babies to sleep on their bellies, actions said to put children at higher safety risk. Other deaths involve children who ingested sleeping pills, were drowned or were injured.

William Fitzpatrick, president of the National District Attorneys Association, based in Alexandria, Va., said that there isn’t a national database to examine what happens to children whose homicides go unprosecuted. However, he is certain that more children are victims of homicide than data suggests. With a sudden infant death, for example, a child could have accidentally suffocated, he said, but there are often not enough clues to confirm that. Even when there’s evidence of wrongdoing, he said, prosecutors don’t always know who did it – particularly if two parents were present.

“The underreported story is the child victims that go unprosecuted,’’ he said. “It’s sadly extremely easy to kill a child.”

When Massachusetts’ child deaths are ruled homicides, police do a pretty good job of closing cases, records show. According to criminologist James Alan Fox of Northeastern University, state police solve 90 percent of homicides involving children under 11 years old, a much higher clearance rate than for murder cases involving older children and adults, according to an analysis of 2000 to 2013 data he carried out for NECIR.

But there is no data in Massachusetts or nationwide to show how many of those homicide investigations led to charges and convictions, said Ryan Backmann, executive director of the Florida-based nonprofit Project: Cold Case Inc. Languishing death investigations mean long delays in identifying and prosecuting perpetrators, Backmann said.

“In the meantime these people are going to have other children,” Backmann said

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The slow pace of the medical examiner’s office can be frustrating to families and law enforcement officials alike, potentially stalling the criminal justice process indefinitely.

Even in the notorious case of Fitchburg preschooler Jeremiah Oliver – who went missing while under state social service supervision -- no cause of death has yet been announced almost two years after his body was found. Jeremiah’s mother and boyfriend, already charged with assault, kidnapping, and child endangerment, could face murder charges if the medical examiner rules the case a homicide.

In some cases, even a finding of homicide does not prompt action. The medical examiner ruled that one-year-old Keanu Ramos of Pittsfield died of “blunt trauma” in February 2010 and the Berkshire County District Attorney’s office confirmed the investigation is still open almost six years later.

But Keanu’s family said they were never even informed that the child allegedly was a victim of homicide. “You have shocked me,” said his great-grandmother, Sandra Mills, when a New England Center reporter informed her earlier this year. She later told her family what she learned and reported back, that they believe his death was natural: “None of us believe it,” she said about the state report.

There’s some indication that state social workers did not know about the medical examiner’s ruling either: DCF didn’t include Ramos on its list of abuse victims. DCF officials declined to talk about the case, but have said generally that medical examiners have not always alerted the agency when a child’s death was linked to abuse and neglect as required by law.

Felix Browne, a spokesman for the state office of public safety, said medical examiners are supposed to notify district attorneys and DCF when a death is ruled a homicide. He would not comment on the Ramos case.

Family members also are waiting for answers in the case of 2-year-old Dean McCullough of Lowell, whose 2010 death was ruled a homicide seven months after his passing, caused by “blunt force trauma of head with injuries to brain,” according to his death certificate. McCullough had an open DCF case at the time of his death and the state child protection agency later determined that his death was linked to abuse and neglect, records show.

But five years later, no one has been charged in Dean’s death. Jennifer Fontes, McCullough’s great aunt, said she is angry and disgusted. “He is literally just forgotten,” she said.

Medeiros feels her grandson was failed in life and in death.

Medeiros said she called DCF multiple times to warn that Jay was at risk after she quarreled with her daughter, who moved out of her North Attleboro home in September 2012.

Two months later, in November, police were called to Richmond Avenue following a 911 call that a baby wasn’t breathing, police records show. The child’s mother met them at the front door and sent them upstairs, where his father was giving Jay CPR, police reported.

Medeiros said she repeatedly called police to investigate and the medical examiner for a death ruling. She believes the couple left after police obtained search warrants to investigate his death two months later.

Worcester police declined to comment, except to say an investigation into Jay’s death is “active.” Redacted records show officials did obtain several search warrants, which Medeiros said focused on Richardson Avenue.

The young couple may have wanted to move on, but their past followed them as they settled into a second-floor apartment on a residential street in Lakewood, Ohio.

In June 2014, the couple was contacted by Cuyahoga County social workers who heard about Jay’s death from the Massachusetts child welfare agency, said county spokeswoman Mary Louise Madigan. The Ohio case was closed in October 2014 after workers visited the family home and determined there was no evidence the baby was unsafe, Madigan said.

Last spring, a New England Center reporter knocked on the couple’s shuttered apartment and left a letter requesting comment when nobody answered. Jay’s father declined to comment.

Medeiros, meanwhile, speaks to anybody she can about her grandson. Her home is filled with Jay’s pictures, her closet stuffed with his toys and clothes, his death certificate clipped to her refrigerator.

She said the District Attorney’s office provides no information. A local police officer has told her privately that without more information, the investigation is stalled. She worries incessantly about Jay’s little brother, now nearly two years old.

“I want justice for Jay,” said Medeiros. “I have to speak for my grandson Jay because nobody else will.”

The New England Center for Investigative Reporting is an independent, nonprofit news outlet based at Boston University and at the studios of WGBH News (NPR/PBS) in Boston. NECIR interns Shan Wang, Bianca Padró Ocasio, Weiwen Zhao, as well as Todd Wallack of the Globe staff contributed research to this project. Jenifer McKim can be reached at jmckim@bu.edu.