(CNN) – They were freed from the notorious Guantanamo Bay detention center in Cuba, only to pick up arms against the U.S. once again, killing Americans overseas.

A U.S. official tells CNN that an estimated 15 freed Gitmo detainees, all released before 2009, and went on to attack Americans in Afghanistan. The number of Americans killed in those attacks is in the single digits. The threat was first brought to light during testimony on Capitol Hill this march.

California Representative Dana Rohrabacher asked the question, “How many lives have been lost by those terrorists who went back to their terrorist activity?” Paul Lewis, of the defense department, responded, “Unfortunately there have been Americans that have died.”

The U.S. has been aware of the high recidivism rate of released Gitmo detainees for some time. In the most recent u-s report, of the 676 detainees released from Gitmo as of January. 118 have returned to the fight and a further 86 are suspected of returning. That’s nearly one out every three released, but as the Washington post first reported, this is the first time u-s officials have confirmed freed detainees targeted and killed Americans.

Lewis continued with, “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, that we should close Gitmo.”

One consistent challenge has come from foreign governments, losing track of the former detainees. However, the political challenge is clear for president Obama, who vowed to close Gitmo just days after he first assumed office.

President Obama said, “I hereby order that Guantanamo Bay will be closed no later than one year from now.” Opposition on Capitol Hill and within the pentagon since then has delayed closure by years. This new information likely to provide more ammunition to critics’ just months before his presidency ends.

Col. Cedric Leighton, CNN military analyst, finished with, “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. And the fact that they’ve killed Americans is most damning of all.”