SAN JUAN, P.R. — The Justice Department on Friday made public the names of 55 detainees at the Guantánamo Bay prison who have been approved for transfer to the custody of other countries, releasing information sought by human rights organizations.

The announcement, which reverses a 2009 decision, was a surprise to organizations that had been seeking the information.

“We did not expect this,” said Omar Farah, a lawyer for the Center for Constitutional Rights. “This is an important development.”

Mr. Farah said the government’s action would aid lawyers representing detainees at the prison. “We can now advocate publicly for the release of our clients by name,” he said.