The hunt for those responsible is continuing, Mueller says in a BBC documentary keyed to the anniversary. The bombing occurred on Dec. 21, 1988.
"We have FBI agents who are working full time to track down every lead, as we have since it occurred 25 years ago," he told the BBC.
"My expectation is that continuously we will obtain additional information, perhaps additional witnesses, and that others will be charged with their participation in this."
The flight from London to New York carried a bomb planted by Libyan agents. It exploded in the air over the town of Lockerbie.
All 243 passengers and 16 crew were killed, as well as 11 people on the ground, making it the deadliest act of terrorism in the U.K., the BBC reported.
The only person ever convicted of the bombing was a Libyan man, Abdelbasset al-Megrahi. He was ultimately released from prison in 2009 on compassionate grounds — he was suffering from prostate cancer — and died last year, still claiming he was innocent.
On his return to Libya, he was welcomed home as a hero and met with then dictator Moammar Gadhafi.
Pan American World Airways went out of business in 1991.
For his part, Mueller says there had been progress since the revolution in Libya and he expects that to continue.
"We do not forget, and by that I mean the FBI, the U.S. Department of Justice; we do not forget."