“Every diocese should assume that it is potentially under investigation,” said Marci A. Hamilton, a law professor and founder of CHILD USA, a think tank at the University of Pennsylvania that studies child sexual abuse and prevention. “Given the crisscrossing paths of priests sent from one diocese to another across the United States, I can understand why someone in Washington would say, ‘O.K., everybody needs to hold onto their evidence right now because we’re just starting to sketch out where the pedophiles have been sent by the hierarchy.’”

Abuse victims have despaired for years that the church in the United States would never be called to fully account for the perpetrators and the people they violated. There have been government-run inquiries in countries like Australia and Ireland, but the federal government in the United States has been reluctant to intervene, until now. The letter from the United States attorney suggested only the potential of a broader investigation, but abuse victims greeted it as a turning point nevertheless.

“It’s going to bring tears to my eyes just thinking about it,” said Tim Lennon, president of the Survivors Network of those Abused by Priests. “It’s hopefully the beginning of a process of long-awaited justice. For survivors, it’s been a long road to bring those who have harmed children and the vulnerable to justice.”

The request to the bishops was contained in a seven-page letter and set of instructions from United States Attorney William M. McSwain, of the Eastern District of Pennsylvania, and sent to the president of the United States Conference of Catholic Bishops, Cardinal Daniel DiNardo of the Archdiocese of Galveston-Houston. The letter, dated Oct. 9, asked that Cardinal DiNardo “immediately transmit copies” to all dioceses in the United States. The bishops conference then sent it to the dioceses on Oct. 23.

Mr. McSwain said in his letter that at this point there was “no need” for the bishops to produce any documents “solely on account of this letter.” He said that the letter was only a request that the church entities “not destroy, discard, dispose of, delete, or alter any of the described documents.” But he started his letter by saying that his office was “investigating possible violations of federal law.”