No evidence has been made public indicating that Mr. Seabrook or others named in the request were involved in any criminality. But the subpoena comes just months after Mr. Bharara sued the city over widespread abuse of inmates and malfeasance at Rikers.

While the lawsuit and an earlier report about jail brutality released by Mr. Bharara’s office last August were never explicitly critical of Mr. Seabrook, an investigation by The Times last year showed that the union president has long been an obstacle to reform efforts at Rikers.

He has resisted stiff punishments for officers charged with excessive force, intervened in investigations and fought stronger screening measures meant to stop guards from smuggling weapons and drugs into jails, The Times found.

For years, Mr. Seabrook, who makes nearly $300,000 annually, has presided with nearly unchecked authority over his union, while becoming close to mayors and governors and exercising extraordinary influence over the city’s Correction Department.

During that time, he has weathered many challenges to his authority. Early in his tenure, he faced allegations of sexual harassment by several women with whom he had worked, settling at least one of the suits. His opponents in the union have been driven out or, in some cases, found themselves the subject of criminal prosecutions.

But Mr. Seabrook has never faced a threat like Mr. Bharara, whose corruption investigations over the last year have led to the indictment of some of New York’s most powerful politicians.

Among the requests in the subpoena is one for any record of “services provided to the union” by Mr. Banks, the former police chief. However it is unclear what, if any, connection he has had to the union. A search of business records by The Times did not turn up any ties to Mr. Seabrook.