"Sean May was a very unique individual," said Adams County District Attorney Don Quick, May's former boss, "and was taken much too soon."

May, an Adams County deputy district attorney, was gunned down as he walked from his car to the back door of his Denver home after a long day at work on Aug. 27, 2008.

His wife Corin was six months pregnant with the couple's first child. She's still not ready to talk about what happened.

Quick tells 9NEWS she and the baby are doing fine. It's a boy who's now nine or 10 months old.

Quick says people in his office talk about May every day.

"I think that the world after August 27, 2008 is a different world for prosecutors, at least at this office, and maybe throughout Colorado," he said. "It used to be that I would tell new hires that as long as you treat folks with respect, you shouldn't have to worry about your safety. I have a tough time saying that now. I don't know what that is."

Investigators are willing to pay $125,000 to anyone who can help them track down whoever killed May.

Quick says May is one of only 11 prosecutors ever murdered in the U.S.

Quick tells 9NEWS he's often questioned if a case he assigned to May lead to the 37-year-old's murder.

"I think I've been asking myself the same questions that other people are. Is there something we could've done different, as the boss?" he said. "I don't blame myself, but I wonder, was I in that causal chain? Did I give him the case that led to him being murdered, or I put him in the spot where that occurred? It's hard being the boss and knowing one of your guys, especially one of the guys that was going to be a dad in a matter of months, was shot down and you wondered did I do something? I've talked to Corin about this and other people and they say, 'Sean wouldn't have had it any other way. He loved his job. He wouldn't have wanted you to back off having him do what you needed him to do.' I take some comfort and solace in that, but you always ask yourself, what if?"

Whoever killed him is still out there, and police say someone must know something.

"We're saying this didn't happen in a vacuum, somebody knows what happened," said Sonny Jackson, a Denver Police Spokesman. "Somebody told somebody what happened. Somebody may have been a witness. That frustrates us when they won't come forward and help serve justice."

Still, Quick says this assault on the justice system won't stop prosecutors from doing their job.
It can't.

"You have to have a justice system you think works. You need to have prosecutors that will do their job regardless of the risk," he said. "Citizens should care about this because if they don't have a justice system then they're not going to have public safety."

Police say the suspect is in his early-to-late 20s, about 5 feet 11 inches tall, with medium build.

At the time, investigators said the man had dark, medium-length hair and a dark-colored moustache and a goatee. He was last seen wearing a backwards, black baseball cap, a white short-sleeved T-shirt, khaki-colored knee-length cargo-style shorts and tennis shoes.

If you have any information about his case, you can give an anonymous tip to Crime Stoppers at 720-913-STOP (7867).