The FBI has found no evidence that the terrorists who blew up Pan American Flight 103 two years ago took advantage of an undercover U.S. drug investigation to plant a bomb on the plane.

The Justice Department said yesterday that the FBI inquiry, initiated last month as part of the Federal Bureau of Investigation's overall probe of the midair explosion, produced nothing to substantiate "recent allegations" of a connection between the bombing and the operations of the Drug Enforcement Administration (DEA).

Pan Am 103 blew up over Lockerbie, Scotland, on Dec. 21, 1988, when a plastic explosive device detonated in a suitcase in the forward baggage hold. All 259 people aboard the New York-bound flight, which originated in Frankfurt, were killed along with 11 on the ground.

NBC and ABC News reported in late October that the terrorists who put the suitcase on the plane may have learned of a DEA operation that used the Frankfurt airport as a point to track heroin shipments from the Middle East to drug traffickers in the Detroit area.

The televised reports said that the shipments apparently were allowed on the Pan Am flights without the usual security checks and that one of the passengers on the plane, Khalid Jaafar, 21, was an undercover DEA informant who may have been unwittingly used to smuggle the bomb aboard.

Deputy Attorney General William P. Barr said that "no evidence has been revealed to support any of the allegations." In addition to the FBI's findings, he said a DEA review showed:

Jaafar "was not an informant nor was he working for the DEA in any other capacity on Dec. 21, 1988, or at any other time. Allegations that Khalid Jaafar visited a DEA office are incorrect."

"DEA had no investigative or operational activity in effect on Dec. 21, 1988, that involved the use of Pan Am Flight 103 or Frankfurt International Airport."

"DEA did not circumvent security measures at the Frankfurt International Airport on Dec. 21, 1988, or at any other time."

DEA denied the reports after they were broadcast, but since the agency was involved in the allegations, the Justice Department ordered a follow-up FBI inquiry to settle the matter.

Law enforcement officials said that as part of ongoing investigations of heroin-smuggling through Lebanon, the DEA in the mid-1980s did make special arrangements with West German authorities to allow undercover officers to carry "controlled" shipments onto U.S.-bound flights from Frankfurt. But the officials said West German police had full knowledge of the shipments and none was made through Frankfurt after 1987.