An official who spoke to CNN Thursday put the numbers at fewer than 15 who went on to attack Americans or coalition forces in Afghanistan. The number of Americans killed in these attacks is believed to be in the single digits, according to the U.S. official, who is familiar with the matter.
"Because many of these incidents were large-scale firefights in a war zone, we cannot always distinguish whether Americans were killed by the former detainees or by others in the same fight," the official said.
Each of the detainees was transferred from the Guantanamo Bay prison before 2009, during the George W. Bush administration. The U.S. believes that nine of them are either dead or in the custody of a foreign government.
The threat was first brought to light in March when a Defense Department official admitted that Americans had been targets of released Gitmo detainees.
"Unfortunately, there have been Americans that have died because of Gitmo detainees," Paul Lewis, the Pentagon's special envoy for Guantanamo detention closure, said at a House Foreign Affairs Committee hearing.
The U.S. has been tracking the numbers of released Gitmo detainees that return to the battlefield.
Of the 676 detainees released from the detention facility as of January, 118 have returned to the fight and a further 86 are suspected of returning, a recidivism rate of nearly 1 out every 3 released, according to the most recent report from the Director of National Intelligence.
But as The Washington Post first reported, this is the first time U.S. officials have addressed the number of former detainees involved in attacks that killed Americans.
"The risk is very high that the hardest of these detainees is going to return to the battlefield and resume their way of warfare," said CNN military analyst retired Col. Cedric Leighton. "Almost nothing that is done at Guantanamo is going to prevent somebody who is a firm believer in jihad against Americans or Western interests from acting against those interests."
President Barack Obama vowed to close Gitmo within a year of assuming office in 2009, but opposition in Congress and within the Pentagon has delayed the closure of the detention facility for years and slowed the rate of transfer of remaining prisoners. The administration argues that closing Gitmo is the right choice despite the risks to American lives.
Sen. Kelly Ayotte, a critic of Obama's plan to close the detention center, sent a letter to the President Tuesday calling on the administration to release the identities of the former detainees responsible for the American deaths, as well as the exact number of Americans killed.
Ayotte was prompted to write the letter after receiving highly classified comments from the Defense Department in response to her request for more information.
"On May 23, the administration submitted answers to these questions -- but classified them in such a way that even my staff with a Top Secret security clearance could not review the response," Ayotte wrote in the letter.
Lewis, of the Pentagon, had a different perspective.
"We don't want anybody to die because we transfer detainees. However, it's the best judgment and the considered judgment of this administration and the previous administration that the risk of keeping Gitmo open is outweighed," said Lewis. "We should close Gitmo."
However, analysts say the political challenge is clear.
"People on the Hill, people in the American public are going to look at this and they're going to say the minute that you release somebody no matter what the statistics are the risk is too great," said Leighton. "The fact that they've killed Americans is the most damning of all."