Director John T. Morton of U.S. Immigration and Customs Enforcement told a House panel the decision was made without consulting the White House or Homeland Security Secretary Janet Napolitano, The New York Times reported Wednesday.
"There were no mass releases of criminals," Morton said, "just efforts to live within our budget."
He said among the detainees released, 629 had past criminal convictions. Officials had determined most did not pose security risks, and monitoring bracelets or other measures were used to prevent them from fleeing after their release on bond, he said.
Eight immigrants had committed serious offenses and four were rearrested by immigration agents.
Republicans charge the Obama administration freed illegal immigrants with criminal backgrounds as part of its efforts to show impact of sequestration spending cuts that went into effect March 1.
Rep. Trey Gowdy, R-S.C., who heads the House Judiciary Committee's subcommittee on immigration, called the detainee release "a public relations stunt gone wrong" that risked the safety of Americans.
Gowdy said Morton could have found other ways to save.
"You could have easily done that," Gowdy said. "Don't act like you could not have."