A U.S. historian who pleaded guilty to stealing letters written by former Presidents George Washington and Abraham Lincoln was sentenced to 18 months in prison on Friday.

Edward Renehan, 52, pleaded guilty in May to one count of interstate transportation of stolen property and admitted stealing a March 1, 1840, letter by Lincoln and two letters dated August 9, 1791, and December 29, 1778, by Washington.

The letters, part of the personal collection of former U.S. President Theodore Roosevelt, were taken from the Theodore Roosevelt Association. Renehan had been the acting director of the New York-based historical and cultural group.

Renehan later sold the letters to a New York gallery for $97,000, the U.S. Attorney's Office in Manhattan said.

Jim Bruns, head of the Roosevelt association, attended the sentencing at Manhattan federal court. He told reporters the letters were"not significant to the shaping of America," but were treasured by Roosevelt, who displayed them in his library.

In sentencing Renehan, who faced a maximum of 10 years in prison, U.S. District Judge Denny Chin called the case"baffling."

"By all accounts, Mr. Renehan is a pre-eminent historian and biographer, and yet he engaged in this stealing," said Chin."It's really hard to understand. I'm not sure that I've heard a convincing explanation."

Renehan, who has written six books including one on the Kennedys, said he has suffered from bipolar disorder, which at the time of the thefts, from 2005 to 2006, was undiagnosed.