Lawmakers on Thursday questioned President Barack Obama’s push to release more detainees from the Guantanamo Bay detention facility, given the recent disappearance of a detainee transferred to Uruguay and that Americans were killed by released detainees who returned to the battlefield.

Paul Lewis, a Defense Department special envoy for the closure of the Guantanamo Bay detention facility, told the House Committee on Foreign Affairs that the Obama administration now estimates 14 released detainees had returned to Afghanistan and were responsible for the deaths of Americans.

Lewis did not provide details on which Americans died from the attacks. But he said all of the former detainees implicated in the killings had been released during President George W. Bush’s administration.

“We’ve got a lot of dead Americans as a result of this catch-and-release program,” said Rep. Brad Sherman, D-Calif., who is a member of the Foreign Affairs Committee.

The Washington Post has reported six Americans have been killed by former Guantanamo detainees since their release.

At its peak, Guantanamo housed 779 detainees. There are 79 now. Most of the detainees were released under the Bush administration. Obama has released 159 detainees and has worked to close the detention facility permanently since he took office.

The Number of Former Guantanamo Detainees Suspected of 'Returning to the Fight' Has Doubled

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Of the 79 remaining prisoners, 29 have been cleared for transfer. The other 50 detainees at Guantanamo could still face judicial punishment, Lewis said.

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