Three members of the Oklahoma National Guard died Thursday in a helicopter crash in southern Iraq.

Seven guardsmen, including four from Texas, were killed in the crash.

Three of the casualties are believed to be Oklahoma guardsmen from Detachment 1, Company B, 2nd Battalion, 149th Aviation from Lexington, Okla., according to a National Guard press release.

“Today we mourn the loss of three Oklahomans who made the ultimate sacrifice,” Gov. Brad Henry said in a press release. “They were courageous men of duty and honor, and our thoughts and prayers are with their families and loved ones during this challenging time.”

Pentagon officials told the Associated Press the crash was most likely caused by a mechanical malfunction, not hostile fire. The CH-47 Chinook helicopter crashed 60 miles west of Basra shortly after midnight en route to the U.S. military’s Balad airbase, north of Baghdad, according to the military.

The military said the seven who died were the only soldiers on board the transport helicopter.

“The brave men and women of the Oklahoma Army and Air National Guard have an awesome responsibility,” Henry said. “They place themselves in harm’s way for the sake of the country they love. The tragic deaths of three Oklahoma guardsmen remind us of the dangers they routinely face so that Americans can enjoy the freedoms upon which this nation was founded.”

An investigation into the crash is ongoing.