The Justice Department plans in coming days to unseal charges against a suspected conspirator in the 1988 bombing of a Pan Am jet over Lockerbie, Scotland, an act of terrorism that killed 270 people and has frustrated American law enforcement officials for more than three decades.

Federal prosecutors in Washington are expected to unseal charges next week against Abu Agila Mas’ud, according to people familiar with the matter who spoke on the condition of anonymity because the charges have not been formally announced. Mas’ud is a longtime suspect in the case who allegedly helped build the bomb that brought down the plane.

A few days before Christmas, Pan Am Flight 103 from London to New York exploded over Scotland, killing all aboard and 11 people on the ground. Most of the passengers were Americans, including a group of college students returning home for the holidays.

Mas’ud has been the subject of discussions among U.S. and foreign officials since at least 2015, when a three-part series on PBS’s “Frontline” named potential suspects in the bombing.

It’s unclear what the likelihood is of the United States taking Mas’ud into custody for trial — his last known location was a Libyan prison where he was serving time for unrelated offenses. The Wall Street Journal and the New York Times reported Wednesday on the coming charges against Mas’ud.

Departing Attorney General William P. Barr, who has about a week left in the job, pushed for a case to be filed against Mas’ud, according to people familiar with the matter. Barr was attorney general in 1991 when he announced the first charges in the case against Libyan intelligence operative Abdel Basset Ali al-Megrahi and his alleged accomplice Lamen Khalifa Fhimah.

FBI agents built their case around the remnants of a Samsonite suitcase that they said held the bomb. That suitcase also carried a shirt bought by Megrahi, as well as a piece of circuitry smaller than a fingernail, which was part of the bomb.

At the time, the Lockerbie investigation was the most complex the FBI had ever undertaken and became a kind of blueprint for future international terrorism probes.

After Megrahi and Fhimah were indicted by the United States, Libya resisted extraditing them for years. In response, the United Nations and the United States imposed stiff economic sanctions and penalties on Libya. In 1999, Libya relented and agreed to an unusual arrangement to turn the men over to be put on trial in the Netherlands before Scottish judges. Megrahi was convicted and sentenced to 27 years to life in prison, while his co-defendant Fhimah was acquitted.

In 2009, Megrahi was released from a Scottish prison on medical grounds after a prostate cancer diagnosis, and he returned home to a hero’s welcome in Libya. Megrahi, who always maintained that he was not involved in the attack, died three years after his release.