The Rev. Shaun Harrison was supposed to be a mentor to troubled kids, a stable source of encouragement and guidance in lives that are racked by instability and hardship. Instead, according to prosecutors, Harrison used his position as a dean at English High School to recruit vulnerable students into a world governed by drugs and guns. Harrison is facing a long list of charges in court this week for allegedly recruiting Luis Rodriguez to sell marijuana for him and then shooting Rodriguez at point-blank range in the back of the head when the teenager wasn't making Harrison enough money back in 2015.

Harrison had some warning signs in his past. He started working for Boston Public Schools in 2010 and had been reprimanded multiple times for pushing a student and making inappropriate comments toward teenage girls. But authorities say his work as an anti-violence activist kept them off their radar as someone capable of the type of violence he allegedly committed. But Harrison, who claimed he had ties to the Latin Kings, kept a treasure trove of firearms and drugs in his Roxbury apartment. The cache of drugs and guns, video evidence and the testimony of the very student he tried to kill are key tools prosecutors are trying to use to put Harrison behind bars. Morning Edition talked to WGBH Legal Analyst Daniel Medwed about the prosecutors' approach and how Harrison plans to present his defense.

Below is a transcript of their conversation:

Joe Mathieu: This is WGBH’s MORNING EDITION.

The trial of Shaun Harrison, a former dean at English High School in Jamaica Plain, is finally underway three years after he was arrested for the attempted murder of one of his own students. Joining us to discuss the Harrison trial is Northeastern law professor and WGBH News legal analyst Daniel Medwed. Morning Daniel, welcome back.

Daniel Medwed: Good morning Joe.

JM: This is one of the most bizarre cases we've seen in a while, some people don't even believe it when they hear the story here. What's the nature of the case against Harrison?

DM: I think bizarre is an apt description. The prosecution's theory is that Harrison hired one of his 17-year-old students Luis Rodriguez to sell marijuana for him. Once marijuana sales started to slump, Harrison allegedly set his sights on killing Rodriguez. So he set up a meeting and they're walking down Magazine Street when Harrison apparently shot Rodriguez in the back of the head.

Amazingly Rodriguez survived, was able to identify Harrison, that led the police to Harrison's apartment where they uncovered basically a treasure trove of guns ammunition marijuana. The upshot is Harrison is facing I believe 10 criminal charges, the most serious of which is essentially armed assault with the intent to commit murder.

JM: Wow. So multiple elements of the story you just told seem unbelievable. How strong is the evidence against Harrison?

DM: This case looks quite strong. First there's the direct evidence, that eyewitness testimony from Rodriguez. He testified late last week, searing powerful evidence. In addition, there's camera footage apparently that partially corroborates the location and events of the incident. Second there's a lot of circumstantial evidence that was obtained from the apartment search. Apparently there is ammunition of the same type and caliber that was at the apartment and also some clothing that matched the description seen on the camera footage that has gunshot residue on it. So it's a pretty strong case or at least it looks like one.

JM: We're talking with Daniel Medwed, WGBH News legal analyst, about Shaun Harrison, the former Boston Public High School Dean on trial now for attempting to murder a student. You just mentioned the trove of evidence including the fact that this was captured on camera. Is it all going to be admitted?

DM: Much of it will be admitted, the eyewitness testimony has already come in, a lot of the physical evidence is due to come in later perhaps this week or early next week. One thing that the trial judge did keep out, however, is that he's barred the prosecution from bringing in any information about Harrison's alleged ties to a notorious criminal street gang.

JM: The Latin Kings would seem to be pretty important or relevant point. Why not?

DM: Yes, on the one hand it's extremely relevant especially because it goes to motive. But on the other hand this also carries the risk of unfair prejudice. If the jury were to learn about the gang affiliation, it could maybe convict Harrison on the basis of that association and be distracted by it and not look closely at the evidence. Judges in every jurisdiction have the benefit of a rule called the rule of discretionary relevance.

They can exclude otherwise relevant or important information if they perceive that it could be substantially outweighed by the danger of unfair prejudice. And I suspect that's what happened here.

JM: Daniel why is this case taking so long, we're talking three years here before going to trial? Could this have happened sooner or maybe be resolved by a guilty plea?

DM: That's a great question. Guilty pleas are pretty ubiquitous in the criminal justice system. More than 90 percent of cases are resolved through pleas. But they're not automatic. First the prosecution has to offer one, the defense then has to accept it and the court has to sign off. So it's unclear to me whether plea negotiations were ever on the table in the Harrison case. But what is clear is that he's adamant right now about mounting a vigorous defense and he's pleading not guilty. What's more this trial was originally slated to go ahead back in 2016 but apparently Harrison had some problems with his lawyer. He claimed that the lawyer wasn't working hard enough and that led to the assignment of a new attorney. Of course that in turn led to ample delays to allow that new person to get up to speed.

JM: So there were case-specific issues here, obviously with that amount of evidence that you talked about do you expect a long trial?

DM: Not necessarily. It all depends on the nature of the defense. Of course the burden is borne by the prosecution to prove guilt beyond a reasonable doubt. The defense doesn't have to do anything but I imagine here Mr. Harrison will try to mount a defense. And that could prolong the trial.

JM: WGBH legal analyst and Northeastern law professor Daniel Medwed. Thanks for the insights as always.

DM: My pleasure, Joe.