The F.A.L.N., which waged a violent campaign for the independence of Puerto Rico, was considered by the authorities to be among the most elusive and resilient terrorist groups to operate in the United States. Among its notable attacks was a bombing at Fraunces Tavern in New York in 1975 that killed four people.

The group was known for its tight-knit membership, fanatical zeal and hit-and-run tactics, as exemplified by the bombings of four government buildings in Manhattan and Brooklyn on New Year’s Eve in 1982 that seriously wounded three police officers.

Mr. Lopez Rivera was not specifically charged in the Fraunces Tavern bombing but more broadly with, among other things, the interstate transportation of firearms with the intent to commit violent crimes, and transportation of explosives with intent to kill and injure people and to destroy government buildings and property.

President Bill Clinton offered Mr. Lopez Rivera and other members of the F.A.L.N. clemency in 1999, a decision that stirred an emotional debate. Mr. Clinton said their sentences were out of proportion with their offenses.

While 12 prisoners accepted the offer and were freed, Mr. Lopez Rivera rejected the chance to reduce his sentence because it did not include all of the group’s members, his lawyer, Jan Susler, said at the time. If he had accepted the agreement, she said, he would have been eligible for release in 2009.