The FBI will review reports that terrorists exploited an undercover drug operation to bomb Pan Am Flight 103 but there is nothing thus far to support them, the Justice Department says.

Deputy Attorney General William P. Barr said Thursday in a statement that "at this time we know of no evidence giving credence to these allegations" but said it was important "to determine their credibility as soon as possible."The department said the FBI will review reports by NBC-TV and ABC-TV that terrorists penetrated a Drug Enforcement Administration operation to put explosives aboard the 1988 flight that ended in a bomb explosion over Lockerbie, Scotland.

In Germany, an official also said investigators would examine the reports but added that an earlier inquiry had failed to substantiate them.

Frankfurt City Prosecutor Hubert Harth said investigators there had examined the charges a year ago. He said the information then pertained not to the DEA but to a drug-tracing operation of the Central Intelligence Agency.

"There was absolutely no confirmation in this direction," Harth said. "Nonetheless, we will once more examine these new reports."

The networks reported Tuesday that terrorists might have substituted baggage containing bombs for suitcases loaded with heroin that the DEA had placed on U.S.-bound planes to track the routes of Lebanese drug traffickers.

FBI Director William Sessions, speaking to reporters in Glasgow, Scotland, refused to comment on the network reports.

Sessions expressed confidence that the bombers would be brought to justice but indicated that the investigation could take additional years.

"We will not let go. We are not only tenacious, but we are diligent about our responsibility," he said.

The FBI investigated similar reports a year ago and found no basis for them, said officials who spoke on condition of anonymity.

NBC reported that terrorists may have discovered an arrangement between DEA and German security officials that allowed them to substitute explosives-filled suitcases for luggage that contained heroin.

The bombing killed 259 people aboard the U.S.-bound flight and 11 more on the ground.