An inmate scheduled to be released from federal prison in 18 months will spend an additional 20 years behind bars for trying to strangle the first black prison guard he had the opportunity to attack, prosecutors said Wednesday.

Morgan Siler, 28, of Portsmouth, Va., was being temporarily held at the federal penitentiary in Atlanta in June 2008 when he attacked a corrections officer. Siler had been transported to Atlanta from a lockup in Florida and was awaiting a transfer to a prison in Virginia, where he was convicted of a series of convenience store robberies.

According to prosecutors, the Atlanta corrections officer, who was not identified, was unlocking cell doors on June 2, 2008, so inmates could get their breakfast when Siler approached the officer from behind with a handmade rope and proceeded to strangle him. The inmate strangled the officer for 35 seconds before another officer came to his aid and the officer was able to be free of the inmate's grip.

Prosecutors said evidence revealed that Siler, who is white, had never met the black corrections officer but vowed to attack the first black guard he saw that day.

"Here, the defendant committed a premeditated and unprovoked attack on a corrections officer," U.S. Attorney Sally Quillian Yates said. "His actions have earned him a significantly more time in prison."

Siler would have been released from prison in February 2014 on the robbery conviction. His 20-year sentence following his June 1 conviction on the new charge will begin after he serves out his current sentence.