The Drug Enforcement Administration said last night that it is investigating reports that terrorists exploited a DEA undercover operation in the Middle East to plant the bomb that blew up Pan Am Flight 103 over Lockerbie, Scotland.

Officials said there was no evidence to substantiate the news reports, but they confirmed that an inquiry was underway.

The DEA statement followed an NBC News report that the terrorists who planted plastic explosive aboard the flight may have learned of the DEA operation, code-named "Operation Courier," and its use of the Frankfurt airport as a point to track heroin shipments to Lebanese drug traffickers in the Detroit area.

"Informants would put suitcases of heroin on the Pan Am flights, apparently without the usual security checks . . . through an arrangement between the DEA and German authorities," NBC-TV reported.

A 21-year-old man from suburban Detroit, Khalid Jaafar, who Pan Am 103 investigators suspected was a drug courier, was once believed to have unwittingly checked a brown Samsonite bag with a bomb in it onto the Pan Am flight at Frankfurt, where it originated. Jaafar was killed in the explosion along with all 258 other people on the plane and 11 on the ground in Lockerbie.

According to the NBC report, terrorists may have discovered that suitcases with heroin could easily get onto U.S.-bound flights from Frankfurt, as part of the DEA operation, and substituted luggage with the bomb for one containing heroin.

DEA spokesman Frank Shults said: "Although no evidence has surfaced to substantiate such claims, we are conducting an inquiry into these allegations, including a review of case files and DEA operations and activities in the relevant time period."

"We have a sincere desire to determine the validity of these allegations -- not only as a responsible federal law enforcement agency, but also out of compassion for the victims of Pan Am Flight 103 and their families," Shults said.

The flight began Dec. 21, 1988, with a Pan Am 727 traveling from Frankfurt to London's Heathrow airport, where its passengers and luggage were transferred to a Pan Am jumbo jet bound for New York.

Investigators found that the disaster was caused by plastic explosive hidden in a Toshiba radio-cassette player and packed into a Samsonite bag at the Frankfurt airport.

Jaafar, a Lebanese Shiite moslem, was suspected early on as a "mule," a drug courier who had unknowingly brought the bomb aboard. He was returning home to Dearborn, Mich., after visiting his native Lebanon and friends in Germany.

According to the NBC report, the DEA's Operation Courier involved undercover infiltration of Lebanese heroin smugglers in the Middle East and their connections to the Detroit area.

After the bombing, investigators questioned Jaafar's parents in Dearborn, his friends in Frankfurt and others, including the travel agent who sold him his ticket. But the evidence indicated that he had only two small carry-on bags with him when he boarded the plane.

Both bags were discovered near Lockerbie, almost intact, indicating that he had carried them on board. The checked bags in the luggage compartment where the bomb had been were shattered. There were reports that Jaafar did check some luggage onto the flight, but these went unresolved.

Investigators believe that the bombing was ordered by Iran in retaliation for the downing of an Iranian airbus over the Persian Gulf and orchestrated by the Popular Front for the Liberation of Palestine-General Command. But according to the New York Times, recently discovered evidence suggests that the Syrian-based PFLP-GC may have backed off at the last minute, because of a raid by police on its West German cell, and enlisted Libyan agents to carry out the operation.

U.S. officials have refused to comment on that report.

The DEA inquiry is being conducted by David Westrake, assistant administrator in charge of internal investigations, and may be completed within a week.