The Drug Enforcement Administration terminated an undercover drug investigation using the Frankfurt airport at least one year before the 1988 terrorist bombing of a Pan American World Airways flight over Lockerbie, Scotland, law enforcement officials said yesterday.

The officials also said that contrary to televised reports earlier this week, there is no record in DEA files that Khalid Jaafar, a 21-year-old Lebanese American who died in the bombing, had been an undercover informant or courier for the agency.

Despite the DEA findings, the Justice Department announced that the FBI would pursue reports that there may have been a connection between DEA undercover operations and the bombing that killed 270 people.

Reports that terrorists may have exploited DEA's special access at the Frankfurt airport and somehow tricked Jaafar into getting the bomb onto the flight were broadcast earlier this week by NBC and ABC. The television networks said Jaafar had been recruited by the DEA to ferry heroin to the United States as part of an undercover operation targeting drug traffickers in the Detroit area.

"While at this time we know of no evidence giving credence to these allegations, it is important . . . to determine their credibility as soon as possible," Deputy Attorney General William Barr said in a prepared statement. He said the FBI's "current involvement with the Pan Am investigation will speed this process."

The statement by Barr rankled DEA officials, who had earlier expressed confidence that their own review would lay the matter to rest. Administration officials said the decision to have the FBI investigate an alleged DEA connection to the bombing was prompted by Attorney General Dick Thornburgh's displeasure with the way the drug agency handled an earlier internal inquiry into the abduction of a Mexican doctor from Guadalajara this year.

In that case, Thornburgh, relying on DEA assurances, told the Mexican government that the agency had no role in seizing the doctor, Humberto Alvarez Machain, and bringing him to the United States for trial on charges of taking part in the 1985 torture-murder of DEA agent Enrique "Kiki" Camarena. But at a federal court hearing in Los Angeles last June, federal prosecutors disclosed that top DEA officials had approved payments to Mexican operatives who abducted Alvarez Machain from his Guadalajara office.

Administration sources said White House and State Department officials had been "upset" with the DEA's handling of that episode and complained to Thornburgh. One government official said there had been "a loss of confidence" by top Justice Department officials in the DEA's ability to investigate itself.

Pan Am Flight 103 originated in Frankfurt on Dec. 21, 1988, with a hop to London, where passengers and luggage were transferred to a Pan Am jumbo jet bound for New York. All 259 people on the plane and 11 on the ground were killed when a bomb in a forward baggage hold detonated. The device had been built into a radio-cassette and packed in a suitcase loaded at Frankfurt.

Administration officials expressed widespread skepticism at the alleged DEA connection and said it smacked of a similar report involving the Central Intelligence Agency and the Frankfurt airport, which the FBI has examined and rejected.

A presidential commission that investigated the bombing put much of the blame on inept and confused Pan Am security in Frankfurt and London. In a report last May, the commission said there was ample evidence that an "extra" bag was put on the plane at Frankfurt. Investigators believe it came from an Air Malta flight.

As part of ongoing investigations of heroin smuggling through Lebanon, DEA in the mid-1980s made special arrangements with West German authorities to allow its undercover operatives to carry "controlled" shipments of drugs onto U.S.-bound flights from Frankfurt, officials acknowledged yesterday. But they said there is no record that any such shipments were made through Frankfurt after 1987.

Officials also said a preliminary check of DEA computers and files turned up no sign that Jaafar had ever worked for the agency. "He's never been documented as working for us, period," said one DEA official. "That doesn't mean he may not have come into our office or talked to some of our people."

New York lawyer James B. Weidner, who was executive director of the presidential panel, said yesterday that his group asked U.S. agencies, including the DEA, specifically whether there was any intelligence or law enforcement operation at Frankfurt that might have had a connection to Pan Am Flight 103 and was told there was none.

"If it {a special arrangement between DEA and West German authorities} were in existence in December of 1988, I would expect to have heard about it," Weidner said. "I don't want to go beyond that."