“A jury is not going to believe a witness who’s also involved in the criminal activity if there isn’t corroboration,” Kelly said. Corroboration could be testimony from other witnesses, documents, or video or other recordings, he said.

Bulger was accused of participating in 19 murders, but the jury found “not proven” or made no finding in all eight murders in which there was just one uncorroborated witness, said Kelly, now a partner with the Boston law firm Nixon Peabody.

Kelly said Martorano could admit from the witness stand that he participated in murders because he had been given “use” immunity under which the government cannot later use anything he said against him or anything derived from what he said.

If evidence of a murder surfaced that Martorano withheld, he could be prosecuted for that, Kelly said.

According to a memorandum Bob McDonnell’s lawyers filed last month, the government entered into an agreement with Williams on July 1, 2013, giving him “transactional” immunity in exchange for his testimony against the McDonnells.

Transactional immunity is a broader protection than use immunity. It protects a witness from prosecution for the crimes at issue, not just against the use of his or her own immunized testimony.