WASHINGTON (AP) — The Justice Department announced new charges Monday against a Libyan bombmaker in the 1988 explosion of Pan Am Flight 103 over Lockerbie, Scotland, an attack that killed 259 people in the air and 11 on the ground.

The charges were revealed on the 32nd anniversary of the bombing and in the final news conference of Attorney General William Barr's tenure, underscoring his personal attachment to a case that unfolded during his first stint at the Justice Department. He had announced an earlier set of charges against two other Libyan intelligence officials in his capacity as acting attorney general nearly 30 years ago, vowing that the investigation would continue. Though Barr had not appeared at a press conference in months, he led this one two days before his departure as a career bookend.

In presenting new charges, the Justice Department is revisiting a case that deepened the chasm between the United States and Libya, laid bare the threat of international terrorism more than a decade before the Sept. 11 attacks and produced global investigations and punishing sanctions.

The case against the alleged bombmaker, Abu Agela Mas'ud Kheir Al-Marimi, is for now more theoretical than practical since Masud is not in U.S. custody and it is unclear if he ever will be, or if the evidence will be sufficient for conviction. But it nonetheless represents one of the more consequential counterterrorism announcements from the Trump administration Justice Department.

“At long last, this man responsible for killing Americans and many others will be subject to justice for his crimes," Barr said.

A breakthrough in the investigation came when U.S. officials in 2017 received a copy of an interview that Masud, a longtime explosives expert for Libya's intelligence service, had given to Libyan law enforcement in 2012 after being taken into custody following the collapse of the regime of the country's leader, Col. Moammar Gadhafi.

In that interview, U.S. officials said, Masud admitted building the bomb in the Pan Am attack and working with two other conspirators to carry it out. He also said the operation was ordered by Libyan intelligence and that Gadhafi thanked him and other members of the team after the attack, according to an FBI affidavit filed in the case.

While Masud is now the third Libyan intelligence official charged in the U.S. in connection with the Lockerbie bombing, he would be the first to stand trial in an American courtroom.

After Barr in 1991 announced charges against the two other men, Abdel Baset Ali al-Megrahi and Lamen Khalifa Fhimah, the Libyan government balked at turning them over to the U.S. The country ultimately surrendered them for prosecution before a panel of Scottish judges sitting in the Netherlands as part of a special arrangement.

Al-Megrahi was convicted while Fhimah was acquitted of all charges. Al-Megrahi was given a life sentence, but Scottish authorities released him on humanitarian grounds in 2009 after he was diagnosed with prostate cancer. He later died in Tripoli.

An FBI affidavit unsealed Monday says Masud told Libyan law enforcement that he flew to Malta to meet al-Megrahi and Fhimah. He handed Fhimah a medium-sized Samsonite suitcase containing a bomb, having already been instructed to set the timer so that the device would explode exactly 11 hours later, according to the document. He then flew to Tripoli, the FBI said.

Masud remains in custody in Libya, but Barr said the U.S. and Scotland would use “every feasible and appropriate means" to bring him to trial.

The New York-bound Pan Am flight exploded over Lockerbie less than an hour after takeoff from London on Dec. 21, 1988. Among the 190 Americans on board were 35 Syracuse University students flying home for Christmas after a semester abroad.

The attack was the latest flare of tension between Libya and the West. In the years before the flight, for instance, Libya was blamed for the bombing of a West Berlin discotheque that killed two American soldiers and injured dozens of others. The complaint against Masud admitted to being involved in that bombing as well.

It wasn’t until 2003 that Gadhafi and Libya formally accepted responsibility for the Pan Am disaster, with the country offering a compensation deal with the victims’ families. Gadhafi agreed to renounce terrorism and weapons of mass destruction, starting a rapprochement that began the following year with the re-opening of embassies. Sanctions were lifted and, in 2006, the Bush administration removed Libya from the U.S. blacklist of state sponsors of terrorism.

The warming of ties did not last long, however, and disintegrated quickly after the Arab Spring began to take hold across the region in 2011. In 2012, the U.S. ambassador to Libya, Chris Stevens and three other Americans were killed on attacks on American diplomatic facilities in Benghazi. The U.S. closed its embassy in Tripoli in 2014.

Besides Barr, another key figure in the Lockerbie investigation was Robert Mueller, who was the Justice Department’s criminal chief at the time the first set of charges was announced. Mueller would later become FBI director and special counsel in charge of the investigation into ties between Russia and the 2016 Trump campaign.

The Russia probe produced a rift between the men after Mueller complained to Barr that he had mischaracterized the gravity of his team’s findings in a letter that he made public before the investigative report was released.

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Associated Press writer Matthew Lee contributed to this report.