Report: Letter says 470 safety inspection reports have not yet been filed with government agency.

NEW YORK (Reuters) - Federal aviation regulators are investigating possible maintenance problems at Northwest Airlines since mechanics went on strike Aug. 20, the Wall Street Journal reported Tuesday.

According to the newspaper, the Federal Aviation Administration said one of its inspectors, temporarily re-assigned from his duties at Northwest, had complained to the FAA and to Mark Dayton, a Democratic senator from Minnesota.

The senator in turn contacted the FAA and the Department of Transportation's Office of the Inspector General and sent a letter to FAA Administrator Marion Blakey, outlining the inspector's assertions, the Journal said.

Dayton asked Blakey to "assure me and the public that Northwest Airlines is meeting all necessary standards of reliability and safety."

Northwest (Research) said it could not comment on the findings because they were not available but said it was in contact with FAA officials. "Safety is of paramount importance to every employee at Northwest Airlines," the company said in a statement.

The Journal said the letter to Blakey alleged that about 470 FAA inspector reports on Northwest's maintenance operations for 11 days after the strike began had not been entered into an electronic database, "which would have triggered a risk assessment."

The letter said that 58 percent to 90 percent of the inspector reports cited defects, compared with a defect rate of 3 percent to 5 percent for Northwest prior to the strike. According to the letter, a 9 percent defect rate would trigger an internal FAA alert.

The airline is using non-union replacement technicians to maintain its fleet after union mechanics walked off the job.