Paul Lewis, the Pentagon's special envoy for Guantanamo detention closure, declined to provide the GOP-led House Foreign Affairs Committee with details. He would not say whether the incidents occurred before or after President Barack Obama took office in January 2009.
"What I can tell you is unfortunately there have been Americans that have died because of (Guantanamo) detainees," Lewis said during an exchange with Rep. Dana Rohrabacher, R-Calif.
"When anybody dies, it is tragedy and we don't want anybody to die because we transfer detainees," Lewis said.
An Obama administration official said Lewis was referring to an incident that involved an Afghan prisoner released from Guantanamo while George W. Bush was president. The official was not authorized to speak publicly and requested anonymity.
During the Bush administration, 532 prisoners were released from Guantanamo, often in large groups to Afghanistan and Saudi Arabia - the two nationalities that made up the greatest number of prisoners.
The Obama administration has released 144 detainees after a screening process that involves representatives from six government agencies and departments who must make a unanimous decision to release.
Lewis testified before the committee along with Lee Wolosky, the State Department's special envoy for Guantanamo closure. They argued the prison is a powerful propaganda tool for the Islamic State group and keeping it open damages U.S. national security.
Republicans and a few Democrats in Congress have repeatedly thwarted Obama's effort to close the prison and blocked any attempt to move detainees to U.S. prisons in legislation the president has signed into law.