The Justice Department has begun an investigation in Los Angeles into allegations that millions of dollars worth of bogus helicopter parts were made in the United States and sold at an enormous profit to unsuspecting military buyers overseas.

Assistant U.S. Attorney Julian Greenspun confirmed that the investigation was under way into the allegations by a knowledgeable industry informant. Greenspun said he was investigating "possible violations" of federal laws but declined to elaborate.

The Federal Aviation Administration and the British Ministry of Defense as well as the Bell and Sikorsky helicopter companies are also conducting their own probes into the charges by the informant, whose identity has not been made public but is known to the federal and private investigators.

The FAA in Seattle, meanwhile, said that tests of some bogus jetliner parts sold to five U.S. airlines in another, apparently unrelated scheme, showed that the parts did not meet FAA safety standards. The parts had been sold with false Boeing aircraft identification and misleading FAA certification, the FAA said.

While a King County, Wash., criminal investigation and an FAA inquiry continue, four of the five airlines said yesterday they had either removed all of the suspected parts from their Boeing 727s and 737s or would soon begin to. The FAA said Monday that a Bellevue, Wash., firm had sold various quantities of 28 different parts that were never certified for safety and were installed on up to 100 jetliners.

Saying "there is no evidence of an immediate hazard" from use of the parts, the FAA gave the airlines up to 45 days to remove them.

The investigation in Los Angeles centers around allegations that Aviation Sales Corp., a New York City firm with subsidiaries in Los Angeles and England, had low-priced and poor-quality parts made in the United States. These parts were sold as highly engineered and top-price equipment to Westland Helicopters Ltd. in Yeovil, England, according to the allegations, and eventually made their way into military helicopter fleets around the world.

In a statement Westland said it was investigating the allegations but said it had found "nothing to substantiate the allegations at all" up to now.

But in an interview today with The Washington Post a former employee of Aviation Sales in Los Angeles said she had taken part in the alleged forgery scheme at the direction of several senior Aviation Sales officials.

The former employee, who asked not to be identified but whose identity is known to federal authorities, said she handled the paperwork for thousands of parts that went from Aviation Sales in California to Westland.

At the direction of top Aviation Sales officials, she said, she had changed identification numbers on the paperwork accompanying the parts to make them appear to have been made for Sikorsky and up to Sikorsky specifications. This was done, according to documents obtained by The Washington Post, because Westland would accept only properly certified Bell or Sikorsky material.

When the bogus Sikorsky parts arrived in England they appeared to bear the proper identification but in fact they were never made or tested to Sikorsky specifications, several former Aviation Sales employees in a position to know have told The Post.

Other knowledgeable sources have told The Post and several investigators about a complex transfer system for similarly manufactured parts which allegedly received false Bell helicopter company identification as well as inaccurate FAA and Bell airworthiness certifications at the direction of Aviation Sales.

Dewitt Larson, regional attorney for the FAA in Los Angeles, said today his office is investigating the charges and wound only add, "We're trying to weigh the validity of the allegations."

But Larson acknowledged that the FAA received a complaint two years ago that Aviation Sales' subsidiary in Chatsworth, Calif., was allegedly manufacturing "defective or non-FAA-regulated parts."

Larson said the FAA had gotten a letter asking for an investigation into whether the company was making bogus parts. "We went to them and asked them and they said no and that was the end of it," Larson said.

In Seattle, the FAA identified five U.S. airlines - American, Braniff, Frontier, Southwest and Wien Air Alaska - as having purchased the bogus Boeing parts from ADS Supply Co.

Braniff said yesterday it had purchased five of each of two parts, a landing gear accessory and a windshield heating unit for cockpits but that all of the parts had been removed by last week. It said no more than five of its 90 Boeing-727 tri-jets had been affected.

A Braniff spokesman said that preliminary tests performed by Boeing on the phony parts showed they were "excellent" and "met standards."

But the FAA's regional counsel in Seattle, Jonathan Howe, said he had not heard that and had received information "tending to show the contrary" - that some tested parts did not meet standards. He could not say, however, whether they were the same parts that Braniff was talking about.

American Airlines said it had purchased two electronic blackboxes for landing gear anti-skid devices from ADS and nine cockpit heating units. All have been removed, a spokesman said, from an undetermined number of 727s.

Frontier Airlines said it has 21 twin-jet 737s and that "most of those" have from one to three electronic devices from ADS. Replacement will start right away, a spokesman said, during normal maintenance and no flight delays would occur.

A Southwest Airlines spokesman said that the company "will do whatever the [the FAA officials] say as soon as we can."

Efforts to obtain comment from Wien Air Alaska were unsuccessful.

The FAA had said Monday that a sixth U.S. airline, Delta, may also have installed the parts, but a spokesman said yesterday Delta has determined it did not purchase any.

Throughout the disclosures, the FAA has emphasized that while the ADS parts were never certified as safe by FAA representatives they are not necessarily an imminent hazard. Some of the parts sold by ADS are non-critical ones - that is, a plane could fly without them - and may also have back-up systems.