Lockerbie bombing case still not closed, former FBI chief says

WASHINGTON -- When a bomb blew apart Pan Am #103 over Scotland, it was the deadliest terror attack ever against U.S. civilians. At the Justice Department, a young prosecutor, Robert Mueller, was tasked with leading the investigation.

Twenty-five years later, Mueller says, the hunt for the bombers goes on.

"This is a case that needs further investigation, and … there are a number of people that we are still seeking," he says. "And hopefully we'll have sufficient evidence to charge in the future."

Investigators quickly determined an explosive device blew a hole in the side of the Boeing 747. Analysts from the CIA traced a tiny bomb fragment to Libya, then headed by strongman Moammar Qaddafi.

Two men were arrested, but only one was convicted. Mueller says others who helped plan and carry out that attack are still at large.

CIA traced a tiny bomb fragment to Libya. CBS News

"You want to know who ordered it," he says. "And you want to know who approved it and who else was involved in carrying out the attack."

Mueller says he believes the planning of the attack took "at least several months."

  "It was well thought out, it was well planned and it was well executed," he says, adding there is no doubt in his mind it went to the highest levels of the Libyan government and that Qaddafi himself may have known about it.

Mueller went on to lead the FBI for 12 years, directing the investigation into the 9/11 attacks. But Pan Am 103 still haunts him. He recalls visiting a storage facility that housed belongings of the victims.

Robert Mueller was tasked with leading the investigation. CBS News

"And you walk along and on these wooden shelves you see a Syracuse sweatshirt, you see some tennis sneakers that were worn by some kid," he says. "And that brings to life the fact that these are lives that are lost that should not have been lost."

Through the years, Mueller has joined families in honoring the victims. And on this anniversary, they will gather once more, mindful that 25 years later, justice has not been done for those lost at Lockerbie.