Two Army helicopters crashed at Joint Base Lewis-McChord in Thurston County, Washington, on Monday night. The Army said all four soldiers on board were killed.
It is not yet known whether the aircraft collided or went down separately, he said.
"We don't have details on what actually occurred," Mathews said. "That will be part of the investigation."
The skies were clear at the time of the crash, according to the Seattle Times.
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Debris was spread over a large area of the base, reported the Tacoma News Tribune. The site was later blocked off by sheriff's deputies and military personnel.
The soldiers killed will not be publicly identified until their relatives have been notified.
An investigation has been launched, to be led by the Combat Readiness Center at Fort Rucker, Alabama.
"Our thoughts and prayers go out to the family, friends and loved ones of the soldiers involved in this tragic accident," said Major General Lloyd Miles, acting senior Army commander at Lewis-McChord, in a statement. "We will conduct a thorough investigation into this incident, and we will do everything in our power to support the families of the brave soldiers who died."
Lewis-McChord, a joint Army and Air Force facility, is one of the largest bases in the US with more than 40,000 military personnel plus their families, civilian staff, contract employees and retirees.
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