The investigation is being conducted by the Office of Children and Family Services, a state agency that oversees juvenile detention centers; the New York State Police; and the Fulton County district attorney. They will seek to determine the cause of the boy’s death, including whether the restraint that was used played a role.

Officials would not identify the boy, citing confidentiality rules for minors.

“For some reason this young man was particularly agitated,” Louise K. Sira, the Fulton County district attorney, said in an interview yesterday, adding that the boy was “physically aggressive, punching and pushing staff members” at the Tryon Residential Center for Boys in Johnstown.

She said the staff members had described using restraint techniques “according to policies and procedures,” but that about 10 minutes later, the boy was “lying down and unresponsive.”

He was taken to a hospital in nearby Amsterdam, where he was pronounced dead, Ms. Sira said.

The case has raised interest among civil libertarians and corrections system watchdog groups, in part because a sharply critical report released in September by Human Rights Watch and the American Civil Liberties Union focused on conditions at an adjacent girls’ section of the Tyron Residential Center and at another girls detention unit at the Lansing Residential Center, near Ithaca.

The report outlined a variety of abuses by staff members at both units, including sexual harassment and violent restraint measures for minor offenses. It leveled criticism at the use of “face-down restraint,” in which it said girls were forced down on the floor with their arms pulled behind them as they were held or handcuffed.

Speaking of the 15-year-old’s death, Donna Lieberman, executive director of the New York Civil Liberties Union, said yesterday, “This death occurred in a facility that is related, the male counterpart, to one of the facilities that the A.C.L.U. investigated quite substantially.”

Mishi Faruqee, director of the Juvenile Justice Project of the Correctional Association of New York, an advocacy group, said yesterday that although the cause of the Bronx boy’s death remained unknown, “we’ve had longstanding concerns” about the use of “prone restraints and the situations where they are used” in state juvenile centers.

Brian Marchetti, a spokesman for the state agency, said its policy on the use of physical restraints was to use “the least aggressive method.” He declined to comment on the type of restraint that was used on the Bronx boy.

“We only allow the use of restraint to prevent youth or staff member injury,” he said, adding that the state agency “has zero tolerance for any misconduct from employees or residents.”

Ms. Sira said, “I don’t know the name of the restraint that was used.” She said, though, that the staff members said the procedure was “right out of the manual” and was “how they’ve been trained.”