The program, officially called the Witness Security Program, is run by the U.S. Marshals Service. Since 1971, it has relocated about 8,600 witnesses and 9,900 family members of witnesses, according to the agency.

The program has protected terrorists and turncoats, kingpins and peons.

Among the people who have spent time in the program: the Mob’s Salvatore Gravano, a close associate of John Gotti; Jamal Al-Fadl, an Al Qaeda defector turned cooperator; Frank Lucas, whose drug dealing in Harlem and Newark was chronicled in the film “American Gangster”; and Henry Hill, the Brooklyn-born wise guy and inspiration for the movie “Goodfellas.”

But Mr. Hernandez’s potential relocation may be challenging.

“It is unlikely” that the Marshals “would pay for the removal of Mr. Hernandez’s signature face tattoos,” Ms. Watkins reported. And “there is almost nothing in Mr. Hernandez’s background that suggests a capacity for discretion.” (The rapper is known for his antics on social media. In one radio interview, he said, “I tell everyone I’m a troll.”)

It is also possible, but rare, to be kicked out of witness protection.

Mr. Hill was “expelled” from it in 1987 “for relentless misbehavior that included drug possession,” The Times reported in a 2012 obituary. (It went on to note that “after leaving witness protection, Mr. Hill lived in comparative safety: most of those he feared were either dead or in prison.”)

Most others have successfully disappeared.

Nicky Barnes, a drug dealer who was featured on a 1977 cover of The New York Times Magazine under the headline “Mr. Untouchable,” testified against his former associates after spending more than two decades in prison.

He was so under the radar that it took about seven years for his death to be reported.