The hacker, Hector Xavier Monsegur, also helped the authorities dismantle a particularly aggressive cell of the hacking collective Anonymous, leading to the arrest of eight of its members in Europe and the United States, including Jeremy Hammond, who the Federal Bureau of Investigation said was its top “cybercriminal target,” the document said. Mr. Hammond is serving a 10-year prison term…

The whereabouts of Mr. Monsegur have been shrouded in mystery. Since his cooperation with the authorities became known, he has been vilified online by supporters of Anonymous, of which he was a member. The memo, meanwhile, said the government became so concerned about his safety that it relocated him and some members of his family.

“Monsegur repeatedly was approached on the street and threatened or menaced about his cooperation once it became publicly known,” said the memo, which was filed by the office of Preet Bharara, the United States attorney in Manhattan…

At his sentencing in November, Mr. Hammond was prohibited by Judge Preska from naming the foreign governments that Mr. Monsegur had asked him to hack. But, according to an uncensored version of a court statement by Mr. Hammond that appeared online that day, the target list included more than 2,000 Internet domains in numerous countries.