Attorney General Barr announced charges against Abu Agila Masud on Monday
The former Libyan intelligence officer is accused of assembling a bomb that blew up a Pan Am flight 103 en route from London to New York in 1988
The plane exploded over the town of Lockerbie, Scotland, killing 270 people
The charges unsealed by Barr on Monday came on the 32nd anniversary of the terror attack and marked the first US trial related to the case
Barr announced initial charges against two other Lockerbie suspects in 1991
Monday marked his final news conference before stepping down as AG