As students stage a national walkout Wednesday morning over gun violence, senior federal officials are sitting down for an expected grilling from Congress over law enforcement's failure to act on tips about the suspect in last month's school shooting in Parkland, Fla.

The massacre at Marjory Stoneman Douglas High School touched off yet another round of soul-searching and national debate over guns in the U.S. that has drawn in activists, lawmakers and the White House. One major change this time, however, is that Florida teenagers who survived the violence are spearheading the conversation.

The Senate Judiciary Committee officially joined the discussion Wednesday with a hearing that opened with Florida's senators begging their congressional colleagues to act.

"We've had our fill of this in Florida the last two years," said Sen. Bill Nelson, D-Fla., before he listed the many mass shootings the state has endured, including the Pulse nightclub massacre in June 2016 that killed 49 people.

Sen. Marco Rubio, R-Fla., raised his voice as he noted that the alleged Parkland shooter, Nikolas Cruz, "didn't slip through one crack, he slipped through every crack."

"This tragedy was the result of two things: the first is of a multi-systemic failure of government agencies at the federal, state and local levels," said Rubio. "And second because of vulnerabilities in our existing laws that need to be addressed."

Questions about the FBI's actions fell to acting FBI Deputy Director David Bowdich, who is testifying alongside the acting director of the Bureau of Alcohol, Tobacco, Firearms and Explosives, Thomas Brandon, and the head of the National Threat Assessment Center, Lina Alathari.

In his opening statement to senators, Bowdich walked through the timeline of when the FBI knew what about Cruz and the potential threat he posed.

In September 2017, Bowdich said the FBI received an email tip about someone posting on YouTube under the name Nikolas Cruz: "I'm going to be a professional school shooter."

And then in January, the bureau's tip line received a call from a close friend of Cruz's family warning that he had made statements about harming himself and others. The caller said they were concerned about him "shooting up a school," Bowdich said.

The FBI operator who took the phone call found the YouTube comment thread consulted her supervisor, and the investigation was closed without any information being forwarded to any FBI field offices or any state or local law enforcement agencies.

"This is not the kind of news I want to deliver to this committee, to the families, or to the public, but we are committed to transparency in all that we do on behalf of the American people," Bowdich said. "When we make mistakes, we will not hide from them."

Bowdich has said the FBI is conducting a full-scale review of the tip line, which he said received some 765,000 calls, and 735,000 emails, last year. He estimated that 1 in 10 of those calls turns into a lead.

Local law enforcement also reportedly missed red flags. The Broward Sheriff's Office also received a tip about Cruz potentially being a "school shooter in the making," according to the Miami Herald. The newspaper says deputies didn't report the warning.

Senate Judiciary Chairman Chuck Grassley, R-Iowa, said he was frustrated that representatives from the sheriff's office declined to testify on Wednesday.

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