Al Gore, the former U.S. vice-president, has been cleared of allegations he groped and assaulted a masseuse in Portland, Ore., in 2006. (Mark Humphrey/Associated Press)

Former U.S. vice-president Al Gore was cleared Friday of allegations he groped and assaulted a masseuse in a luxury Portland, Ore., hotel room in 2006, closing a case that could have tarnished the Nobel prize winner's reputation.

After a four-week investigation that included interviews with Gore, masseuse Molly Hagerty, her acquaintances and hotel staff, as well as testing a pair of stained pants belonging to Hagerty, Multnomah County District Attorney Michael Schrunk said there was no basis for prosecution.

In a memo to Schrunk released with the decision, Don Rees, the senior deputy district attorney, cited "contradictory evidence, conflicting witness statements, credibility issues, lack of forensic evidence and denials by Mr. Gore."

Rees also said Hagerty and her attorneys were unco-operative, witnesses could not remember anything unusual, Hagerty failed a polygraph examination and she would not say whether she was paid by a tabloid newspaper for her story.

Gore aides welcomed the news.

"Mr. Gore unequivocally and emphatically denied this accusation when he first learned of its existence three years ago," spokeswoman Kalee Kreider said in a statement. "He respects and appreciates the thorough and professional work of the Portland authorities and is pleased that this matter has now been resolved."

Gore and his attorneys met with Portland detectives in San Francisco on July 22, telling them he remembered almost nothing about the woman and was "completely baffled" by her statements, according to the memo.

There were questions about Hagerty's claims from the beginning. She first contacted police in 2006 through an attorney, claiming "unwanted sexual contact" by Gore, but the attorney declined to discuss any details.

Hagerty then failed to attend meetings scheduled three times with detectives, and the attorney finally said it would be handled as a civil complaint.

Nothing further was heard from Hagerty until January 2009, when she appeared at police headquarters to say she wanted to file a criminal complaint.

An interview with a detective resulted in a 67-page transcript describing the massage therapy session with Gore at the Hotel Lucia, including an "inescapable embrace."

The memo noted that investigators determined the claims "did not merit further inquiry" and did not refer it to the district attorney's office.