Isaac Turnbaugh, 28, told police in Randolph, Vermont last month that he shot his 24-year-old co-worker, Declan Lyons, in the head with a rifle in 2002 and wanted to surrender to officials, the Burlington Free Press reported.
But in 2004, Turnbaugh was found not guilty of the first-degree murder of Mr Lyons in a pizza restaurant by a jury.
As such, he could not be charged again for the same death under the double jeopardy law, Attorney General William Sorrell said.
“You only get one bite of the apple. It’s double jeopardy,” Mr Sorrell said.
“You can go out on the courthouse steps and confess, and the state can’t do anything.”
Mr Sorrell said the bullet that killed Mr Lyons was never found. Describing the case as “tough”, he said: “You’ve got to live with the outcome.”
He said in 2004 the jury “chose to believe that someone who repeatedly confessed to the murder was lying”.
Mr Turnbaugh’s lawyers had argued during his trial that he only claimed responsibility for the killing because he was mentally ill.
After Mr Turnbaugh’s confession last month, he was brought to a local police station. He hit a police officer in the face and was later charged with assault, it was reported.
He was held at the Vermont State Hospital and was undergoing a mental health evaluation.