For example, Air National Guard regulations at the time required commanders to write an investigative report for the Air Force when Mr. Bush missed his annual medical exam in 1972. The regulations also required commanders to confirm in writing that Mr. Bush received counseling after missing five months of drills.
No such records have been made public and the government told The Associated Press in response to a Freedom of Information Act lawsuit that it has released all records it can find.
Outside experts suggest that National Guard commanders may not have produced documentation required by their own regulations.
"One of the downfalls back then in the National Guard was that not everyone wanted to be chief of staff of the Air Force. They just wanted to fly or maintain airplanes. So the record keeping could have been better," said retired Maj. Gen. Paul A. Weaver Jr., a former head of the Air National Guard. He said the documents might not have been kept in the first place.
Challenging the government's declaration that no more documents exist, the AP identified five categories of records that should have been generated after Mr. Bush skipped his pilot's physical and missed five months of training.
"Each of these actions by any member of the National Guard should have generated the creation of many documents that have yet to be produced," AP lawyer David Schulz wrote the Justice Department Aug. 26.
White House spokeswoman Claire Buchan said there were no other documents to explain discrepancies in Mr. Bush's files.
Military service during the Vietnam War has become an issue in the November presidential election as both candidates debate the current wars in Iraq and Afghanistan.
Mr. Bush's challenger in the Nov. 2 elections, Democrat John Kerry, commanded a Navy Swift boat in Vietnam and won five medals, including a Silver Star, the U.S. military's third-highest combat medal. But his heroism has been challenged in ads by some veterans who support Mr. Bush.
The president served stateside in the Air National Guard during Vietnam. Democrats have accused him of shirking his Guard service and getting favored treatment as the son of a prominent Washington figure.
The AP talked to experts unaffiliated with either campaign who have reviewed Mr. Bush's files for missing documents. They said it was not unusual for guard commanders to ignore deficiencies by junior officers such as Mr. Bush. But they said missing a physical exam, which caused him to be grounded, was not common.
"It's sort of like a code of honor that you didn't go DNF (duty not including flying)," said retired Air Force Col. Leonard Walls, who flew 181 combat missions over Vietnam. "There was a lot of pride in keeping combat-ready status."
Mr. Bush has said he fulfilled all his obligations. He was in the Texas Air National Guard from 1968 to 1973 and was trained to fly F-102 fighters.
"I'm proud of my service," Mr. Bush told a rally last weekend in Lima, Ohio.
Records of Mr. Bush's service have significant gaps, starting in 1972. Mr. Bush has said he left Texas that year to work on the unsuccessful Senate campaign in Alabama of family friend Winton Blount.
Mr. Bush was approved to train in September, October and November 1972 with the Alabama Air National Guard's 187th Tactical Reconnaissance Group. The only record tying Mr. Bush to that unit is a dental exam at the group's Montgomery base in January 1973. No records have been released giving Mr. Bush permission to train with the 187th after November 1972.
Walls, the Air Force combat veteran, was assigned to the 187th in 1972 and 1973 to train its pilots to fly the F-4 Phantom. Walls and more than a dozen other members of the 187th say they never saw Mr. Bush. One member of the unit, retired Lt. Col. John Calhoun, has said he remembers Mr. Bush showing up for training with the 187th.
Pay records show Mr. Bush was credited for training in January, April and May 1973; other files indicate that service was outside Texas.
A May 1973 yearly evaluation from Mr. Bush's Texas unit gives the future president no ratings and stated Mr. Bush had not been seen at the Texas base since April 1972. In a directive from June 29, 1973, an Air Force personnel official pressed Mr. Bush's unit for information about his Alabama service.
"This officer should have been reassigned in May 1972," wrote Master Sgt. Daniel P. Harkness, "since he no longer is training in his AFSC (Air Force Service Category, or job title) or with his unit of assignment."
Then-Maj. Rufus G. Martin replied Nov. 12, 1973: "Not rated for the period 1 May 72 through 30 Apr 73. Report for this period not available for administrative reasons."
By then, Texas Air National Guard officials had approved Mr. Bush's request to leave the guard to attend Harvard Business School; his last days of duty were in July 1973.