Documents that could have decided a dispute over President George W Bush's days in the military 30 years ago have been destroyed, the Pentagon says.
Mr Bush trained as a pilot in the National Guard
Microfilm containing the pertinent payroll records for the Texas-based Air National Guard had been damaged and could not be salvaged, it said.
Democrats have accused the president of ducking the draft call to Vietnam in favour of less dangerous duties.
The White House has released some records in a bid to refute the charges.
'No back-up copies'
The destroyed files included President Bush's pay records for two three-month periods in 1969 and 1972, a Department of Defence statement said.
"Searches for back-up paper copies of the missing records were unsuccessful," it added.
The 1969 period is not contentious for Mr Bush, as it is already known he was training to be a pilot at the time.
But in 1972 he moved to Alabama to work on a political campaign, and opponents say he failed to turn up for guard duties during this time.
The lost records might have thrown some light on whether he fulfilled his legal commitment.
Doubts were first raised nationally about Mr Bush's service during the 2000 presidential campaign and the issue has resurfaced as Mr Bush fights for re-election.
Democratic presidential hopeful John Kerry, a decorated Vietnam veteran, has said Mr Bush must come clean on what he did.
A White House spokeswoman has said the documents already released prove beyond doubt that President Bush "fulfilled his duties in the National Guard at the time".
The microfilm containing the records apparently disintegrated as staff were trying to preserve it from decay.
The loss was announced by the Pentagon's Office of Freedom of Information and Security Review in letters responding to media demands for full access to Mr Bush's records.
"The Defence Finance and Accounting Service has advised of the inadvertent destruction of microfilm containing certain National Guard payroll records," said the letter, signed by the office's chief C Y Talbott.
It added that in 1996 and 1997 the microfilm records of "numerous service members" from the first quarter of 1969 and the third quarter of 1972 were ruined - Mr Bush's among them.
Mr Bush trained as a pilot while a member of the Texas and Alabama air national guard but never flew in combat.
He left the national guard in 1973 with an honourable discharge to attend Harvard Business School.