The American military helicopter that crashed in Iraq over the weekend, killing all 12 soldiers aboard, was most likely shot down by enemy fire, U.S military officials told FOX News on Monday — the same day an Al Qaeda-linked group claimed responsibility.

Investigators have gathered what one official describes as "quite a bit of evidence" pointing in the direction of the chopper being brought down by an insurgent rocket. However, the U.S. military officially states the cause of the crash has not yet been determined.

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The news bolsters Monday's claim by an Al Qaeda-linked coalition of Iraqi Sunni insurgents that its fighters shot down the chopper.

The insurgent coalition, the Islamic State in Iraq, posted the claim on an Islamic Web site, saying that "the lions of Iraq's Islamic state managed to down a Black Hawk on Saturday, which was followed by a clash with the Crusaders, and that led to the destruction of two Humvees and the annihilation of those inside, thanks be to God."

The posting's authenticity could not be independently verified, but it appeared on a Web site used as a clearing house for militant statements. The Islamic State in Iraq is believed to be the political wing of Al Qaeda in Iraq.

The Army Black Hawk crashed in good weather in Diyala province, northeast of Baghdad. U.S. and Iraqi forces have been battling Sunni insurgents and Shiite militias for months in the province.

A U.S. military spokeswoman, Lt. Col. Josslyn Aberle, said the cause had not been determined. Navy Capt. Frank Pascual, a member of a U.S. media relations team in the United Arab Emirates, told Al-Arabiya television that the helicopter was believed to have suffered technical troubles before going down.

In Washington, a senior U.S. military official said Monday that investigators had found debris near the crash scene that could belong to a shoulder-fired weapon which may have been used to shoot down the helicopter. The official, who requested anonymity because the investigation is ongoing, said there was some belief that the piece of debris was a tube.

Officials were still combing through the debris to make a final determination about the cause of the crash.

The helicopter crash was the fourth deadliest since the start of the war in March 2003.

The Associated Press contributed to this report.