The Air Force said Friday it has punished 70 airmen involved in the accidental, cross-country flight of a nuclear-armed B-52 bomber following an investigation that found widespread disregard for the rules on handling such munitions.

"There has been an erosion of adherence to weapons-handling standards at Minot Air Force Base and Barksdale Air Force Base," said Maj. Gen. Richard Newton, the Air Force deputy chief of staff for operations.

Newton was announcing the results of a six-week probe into the Aug. 29-30 incident in which the B-52 was inadvertently armed with six nuclear-tipped cruise missiles and flown from Minot in North Dakota to Barksdale in Louisiana without anyone noticing the mistake for more than a day.

The missiles were supposed to be taken to Louisiana, but the warheads were supposed to have been removed beforehand.

A main reason for the error was that crews had decided not to follow a complex schedule under which the status of the missiles is tracked while they are disarmed, loaded, moved and so on, one official said on condition of anonymity because he was not authorized to speak on the record.

The airmen replaced the schedule with their own "informal" system, he said, though he didn't say why they did that nor how long they had been doing it their own way.

"This was an unacceptable mistake and a clear deviation from our exacting standards," Air Force Secretary Michael W. Wynne said at a Pentagon press conference with Newton. "We hold ourselves accountable to the American people and want to ensure proper corrective action has been taken."

Rep. Ellen Tauscher, D-Calif., chairwoman of the House Armed Services strategic forces subcommittee, said she believed the Air Force had done a thorough investigation, but the findings were "a warning sign that there has been degradation" of attitudes toward the handling of the weapons.

"These are not just rules that people dreamed up ... just so they could check off the boxes," she said. "This is fundamentally important to the security of the country and the world."

Hans Kristensen of the Federation of American Scientists was among those skeptical that the August flight represented an isolated incident.

He said a decline in Air Force standards for nuclear weapons maintenance and security was documented by the government a decade ago. In recent years, he said, Minot and Barksdale have both gotten poor marks during inspections routinely required for certification.

"Part of the reason is that after the end of the Cold War, and the disappearance of the Soviet nuclear threat, the nuclear career was not very sexy — it was not the way to go if you wanted to" advance in the military. A shortage of people with the right skills, training and mentality followed, something the Air Force has worked to improve, he said.

Newton acknowledged that the Air Force needs to "restore the confidence" lost among the American people after the August incident, which raised questions about the safety of the country's nuclear arsenal.

"We are making all appropriate changes to ensure this has a minimal chance of ever happening again," Wynne said.

Newton said the flight in question resulted from an "unprecedented string of procedural errors," beginning with a failure by airmen to conduct a required inspection of the missiles before they were loaded aboard the B-52 bomber at Minot. The crew flying the plane was unaware nuclear warheads were on its wing, though it wasn't explained what role they played in the mistake.

Highest ranked among those punished were four officers who were relieved this week of their commands, including the 5th Bomb Wing commander at Minot — Col. Bruce Emig, who also has been the base commander since June.

In addition, the wing has been "decertified from its wartime mission," Newton said.

Some 65 airmen have been decertified from handling nuclear weapons. The certification process looks at a person's psychological profile, any medications they are taking and other factors in determining a person's reliability to handle weapons.

After it was loaded with the missiles, the B-52 sat overnight at Minot, flew the next morning to Louisiana, and then sat on a tarmac again for hours before anyone noticed the nuclear warheads.

Newton avoided repeated questions on what extra security would have been required if crews had known the nuclear weapons were on the plane. But another official later said privately that security was increased as soon as the nuclear warheads were discovered.

The Air Combat Command ordered a command-wide stand-down — instituted base by base and completed Sept. 14 — to set aside time for personnel to review procedures, officials said.

The incident was so serious that it required President Bush and Defense Secretary Robert Gates to be quickly informed.

Wynne prefaced his remarks about the B-52 incident by saying that, in publicly confirming that nuclear weapons were involved, he had authorized a one-time exception to U.S. policy, which states that the location of nuclear weapons will never be confirmed publicly. He said he made this exception because of the seriousness of the episode and its importance to the nation.

The weapon involved was the Advanced Cruise Missile, a "stealth" weapon developed in the 1980s with the ability to evade detection by Soviet radar. The Air Force said in March that it had decided to retire the Advanced Cruise Missile fleet soon, and officials said after the breach that the missiles were being flown to Barksdale for decommissioning.