Colombia’s attorney general and justices joined advocates in complaining to the Obama administration, which had inherited the cases, that their pursuit of justice was being thwarted. As a result, in 2010, the Justice Department created an “access plan,” promising to make about a dozen key paramilitaries available for in-person or video interviews — if the men were willing.

Mr. Carr, spokesman for the Justice Department, said that more than 2,000 video depositions and interviews with paramilitary leaders had taken place, and that some had been televised to centers in Colombia from which victims could ask questions.

Still, even in the cases of those leaders like Mr. Mancuso who participated most actively, these sessions were insufficient, according to a Colombian prosecutor who testified last year in his case.

“In total, we progressed maybe 8 percent of what we have to progress,” said the prosecutor, Giovanni Álvarez Santoyo. He noted that Mr. Mancuso had been charged in 4,800 “matters,” from “murder to forced disappearances, through forced removals, recruiting of minors, sexual violence, sexual slavery, and related crimes such as kidnapping, terrorism, theft and destruction of protected property.”

It is impossible to know how things would have been different if the leaders had not been extradited — whether more truth would have been divulged, whether greater or quicker justice would have been done, and whether the paramilitary groups would have been more successfully, or less successfully, dismantled.

Iván Velásquez, who led the Supreme Court’s paramilitary corruption investigation, said at a forum in Bogotá a few years ago that the truth-telling by paramilitary leaders was not impressive before or after the extraditions.

“Many of them reduced a good part of their ‘collaboration,’ as we call their obligation to tell the truth, to relating in a decontextualized manner the murders committed — justifying them by portraying their victims as guerrillas dressed in civilian clothing — and revealing the locations of graves,” he said.