U.S. transportation investigators are looking into allegations of possible maintenance and regulatory oversight problems at Northwest Airlines (NWAC), where replacement workers have stepped in for striking mechanics, authorities said Tuesday.

The Federal Aviation Administration (search) and the Transportation Department (search) inspector general's office were reviewing assertions by an FAA inspector that surfaced after 4,400 mechanics walked off the job on Aug. 20 in a dispute over proposed concessions sought by the financially troubled No. 4 U.S. carrier.

The inspector, who was not identified, has alleged that maintenance defect reports at Northwest were up sharply in the first 11 days of the strike.

The inspector also said crucial inspection data was not being analyzed properly by regulators who had pledged close oversight when the work stoppage began. The airline would not comment on the allegations but said managers were in "constant contact" with the FAA on safety issues.

An FAA spokesman, Greg Martin, said FAA Administrator Marion Blakey had ordered a full investigation into the allegations as well as counterclaims that the inspector was intimidating replacement workers and was openly critical of Northwest management.

"On both issues we have not made any prejudgments. Depending on the merits on one or both charges we will take decisive action," Martin said. He said the inspector had been reassigned pending the outcome of the investigation.

A spokesman for Transportation Department Inspector General Kenneth Mead confirmed investigators from that office had interviewed the inspector for three hours on Friday but no decision had been made on whether to launch an audit separate from the FAA probe.

In its latest update, Northwest said it had completed nearly all of its scheduled flights on Sunday with more than 80 percent arriving on time.

The FAA had not raised any safety concerns at Northwest since the strike was launched.

Transportation and senior aviation authorities in Washington were alerted to the inspector's complaints by U.S. Sen. Mark Dayton, a Minnesota Democrat.

According to a letter from Dayton to Blakey, the inspector said maintenance defects were cited in more than half of nearly 500 reports submitted by FAA inspectors on Northwest maintenance operations between Aug. 20 and Aug. 31.

The inspector also asserted that the information was not entered into an electronic database that tracks maintenance problems and automatically raises alerts that would require tighter oversight of airline operations.

Mead's office issued a report last spring that was sharply critical of FAA monitoring of maintenance at financially troubled airlines, particularly its oversight of outsourced work. The agency disputed the findings.