WASHINGTON — A Libyan intelligence operative charged in the 1988 bombing of an American jetliner over Lockerbie, Scotland, was arrested by the F.B.I. and being extradited to the United States to face prosecution for one of the deadliest terrorist attacks in American history, officials said on Sunday.

The arrest of the operative, Abu Agila Mohammad Mas’ud, was the culmination of a decades-long effort by the Justice Department to prosecute him. In 2020, Attorney General William P. Barr announced criminal charges against Mr. Mas’ud, accusing him of building the explosive device used in the bombing, which killed 270 passengers, including 190 Americans.

Mr. Mas’ud faces two criminal counts, including destruction of an aircraft resulting in death. He was being held at a Libyan prison for unrelated crimes when the Justice Department unsealed the charges against him two years ago. It is unclear how the U.S. government negotiated the extradition of Mr. Mas’ud.

After Col. Muammar el-Qaddafi, Libya’s leader, was ousted from power, Mr. Mas’ud confessed to the bombing in 2012, telling a Libyan law enforcement official that he was behind the attack. Once investigators learned about the confession in 2017, they interviewed the Libyan official who had elicited it, leading to charges.

Even though extradition would allow Mr. Mas’ud to stand trial, legal experts have expressed doubts about whether his confession, obtained in prison in war-torn Libya, would be admissible as evidence.

Mr. Mas’ud, who was born in Tunisia but has Libyan citizenship, was the third person charged in the bombing. Two others, Abdel Basset Ali al-Megrahi and Al-Amin Khalifa Fhimah, were charged in 1991, but American efforts to prosecute them ran aground when Libya declined to send them to the United States or Britain to stand trial.

Instead, the Libyan government agreed to a trial in the Netherlands under Scottish law. Mr. Fhimah was acquitted and Mr. al-Megrahi was convicted in 2001 and sentenced to life in prison.

In 2009, Scottish officials released Mr. al-Megrahi because he had prostate cancer, despite the strenuous objections of the families of the victims and of American officials, including President Barack Obama. Mr. al-Megrahi died in 2012; his family posthumously appealed his conviction in Scotland, but last year a panel of judges refused to overturn it.

Prosecutors say that Mr. Mas’ud played a key role in the bombing, traveling to Malta and delivering the suitcase that contained the bomb used in the attack. In Malta, Mr. Megrahi and Mr. Fhimah instructed Mr Mas’ud to set the timer on the device so it would blow up while the plane was in the air the next day, prosecutors said.

On the morning of Dec. 21, 1998, Mr. Megrahi and Mr. Fhimah met Mr. Mas’ud at the airport in Malta, where he turned over the suitcase. Prosecutors said Mr. Fhimah put the suitcase on a conveyor belt, ultimately ending up on Pan Am Flight 103.

Mr. Mas’ud’s name surfaced twice in 1988, even before the bombing took place. In October, a Libyan defector told the C.I.A. he had seen Mr. Mas’ud at the Malta airport with Mr. Megrahi, saying the pair had passed through on a terrorist operation. Malta served as a primary launching point for Libya to initiate such attacks, the informant told the agency. That December, the day before the Pan Am bombing, the informant told the C.I.A. that the pair had again passed through Malta. Nearly another year passed before the agency asked the informant about the bombing.

But investigators never pursued Mr. Mas’ud in earnest until Mr. Megrahi’s trial years later, only for the Libyans to insist that Mr. Mas’ud did not exist. Mr. Megrahi also claimed he did not know Mr. Mas’ud.