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CLEVELAND- The FOX 8 I-Team has obtained a letter investigators say they got from a convicted killer, and it has sparked an investigation into whether or not he also helped send an innocent man to prison.

Cuyahoga County prosecutors are investigating a letter they say was sent to them and others from Robert Yates. He recently went to prison for 42 years-to-life for a string of violent crimes including a murder in Cleveland.

Yates had been facing the possibility of going to death row before his plea.

The letter says, “I’d like to thank you for the 42-L. Your really a life saver (LOL).”

But it also refers to a shooting nearly a decade ago. “I along with two other men were robbed and shot….I made up a description…they showed me a full book of photos. I picked dude out….He’s been in jail for over 8 years now for nothing.”

Now, Cuyahoga County prosecutors are looking back at the old case to make sure they didn’t put the wrong man in prison.

As for Yates, video shows he robbed a man outside a store on Cleveland’s east side, got into a scuffle with the victim, and then shot and killed him in the street. Cleveland police also tied Yates to a dozen other robberies. Those cases included people held up for their wallets and phones. Investigators say Yates waved a gun and sometimes fired shots. They also say, more than once, Yates robbed people sitting in parked cars. Among those victims, a woman with kids in her car.

What to make of the letter? It ends with, “Just think of all the unsolved murders and felonies. I’ll see you in hell.”

The letter says the man who may have been wrongfully convicted in the old case made a plea deal in exchange for a 20-year sentence. A law enforcement source says the suspect actually served four years for involuntary manslaughter.

Again, Cuyahoga County prosecutors are sorting it all out.