FRESNO, Calif. (KFSN) -- On the 16th anniversary of a Fresno man being stabbed to death, the killer is standing trial for murder and arguing his confession doesn't make it murder.
Andrew Alvarez was popular among his neighbors in central Fresno, so when they didn't see him for a few days, they noticed something was off.
"I said it smells like something is dead or something," said Maria Neri.
They called 911 and when police came, they made a disgusting discovery and found puzzle pieces they couldn't put together for a long time.
"It was 16 years ago today (Wednesday) that Andrew Alvarez's body was found decomposing, stabbed, naked, and wrapped in a futon mattress," said prosecutor Deborah Miller.
One of the first people homicide detectives interviewed in 2005 was Joseph Shepherd.
But the case went cold for nine years until Shepherd was in prison for another crime.
Analysts discovered his DNA in fingernail scrapings from Alvarez.
And this time, Detective Bart Ledbetter got a confession from Shepherd.
"The defendant admitted that he was responsible for the death of Andrew Alvarez," Miller told the jury.
Shepherd described how he did it, how he covered his tracks, and also gave police an explanation, which is now a key part of his defense in trial.
His defense attorney says a relative repeatedly sexually assaulted Shepherd as a child.
He was diagnosed with PTSD in 2017.
Curtis Sok says Shepherd doesn't act reasonably when he's triggered, like when Alvarez walked up to him naked that day in 2005.
"He was shocked," Sok said. "'What is this?' And Mr. Alvarez touched him, made an unwanted advance. Mr. Shepherd pushed him away. That's when the trigger happened."
He says Shepherd acted in fear for his life, so when he stabbed Alvarez, it wasn't first degree murder.
The trial is expected to last about three weeks.
Shepherd faces 51 years to life in prison if he's convicted of first degree murder.