Murder conviction in doubt

>>> back now with a murder mystery and a man locked away for 25 to life for a crime he swears he didn't commit, convicted based largely on eyewitness accounts. the story has taken some twists and turns over the course of a ten-year nbc news investigation. and we get our report from nbc's luke russert .

>> reporter: if you walk through any maximum security prison , you're likely to hear the same thing over and over again. i'm innocent. but every once and a while you hear an inmate's story that makes you wonder, maybe this time it's true.

>> i should not be here because i didn't commit this crime.

>> reporter: meet new york state inmate number aa-2303, john adrian velazquez , convicted of killing a retired new york city police officer during a botched robbery in harlem, a conviction he's always claimed is unjust.

>> it was a tuesday, january 27th , 1998 .

>> reporter: attorneys bob gottlieb and celia gordon represented velazquez for the last two years.

>> there was a number spot, an illegal gambling joint in harlem owned by albert wart.

>> reporter: two men barthed in. there was a struggle and then gunfire. 59-year-old albert ward was killed. shot once in the head. the eyewitnesses gave detectives similar descriptions of the shooter and his accomplice.

>> the gunman was a light skinned male black with braids.

>> lo and behold when they have the lineup, you have john adrian velazquez , who is hispanic.

>> reporter: did you ever have dreadlocks?

>> absolutely not.

>> reporter: has there ever been any physical evidence to link you to this crime?

>> none whatsoever.

>> reporter: you were nowhere close to the scene. why is it that you were picked out?

>> that's probably the best question in the world. i don't know the answer.

>> reporter: velazquez was found guilty of second degree murder.

>> i know i heard my mother. i heard her scream. i don't think i looked back. i don't think i had the heart to look back in my familiar's eyes.

>> reporter: he went to work trying to prove his innocence. "dateline" first heard from velazquez ten years ago when he began writing us letters.

>> i needed to be heard. i needed somebody to look into this. i needed somebody to prove what i've been trying to prove all along. i didn't do this.

>> reporter: no physical evidence has ever linked velazquez to the murder. his conviction came solely from eyewitness identification , something used by many prosecutors all over the country. while velazquez makes a persuasive argument for his innocence, there are many layers to his story and many questions not easily answered as you'll see in our full report tonight.