Two U.S. Army helicopters crashed on Monday night killing all four soldiers aboard.

The crash occurred at the Joint Base Lewis-McChord near Tacoma, Wash. after 8 p.m. local time.

It remains unclear whether the single-engine, four-bladed OH-58D Kiowa Warrior helicopters collided or if the aircrafts crashed separately. Officials have noted that they do not have any details as of yet on what caused the two helicopters to crash.

The crashes occurred during training exercises, according to a statement from the Joint Base Lewis-McChord.

There were no injures reported to anyone on the ground. Initial reports said that two additional soldiers were injured, however, that was not the case.

Police and army officials have blocked off access to the crash site and the Combat Readiness Center at Fort Rucker, Ala. will be leading the investigation into the crashes.

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“Our thoughts and prayers go out to the family, friends and loved one of the soldiers involved in this tragic accident,” said senior Army commander at the Washington base, Major General Lloyd Miles.

Miles indicated that a “thorough investigation” into the incident would be conducted and that the Army would “do everything in our power to support the families of the brave soldiers who died this evening.”

The crash occurred in nearby woods and local media reported that heavy fog was the likely culprit in the crashes.

The names of the dead Army aviators will not be released until their families have been notified.

Over 40,000 military personnel are stationed at the Joint Base Lewis-McChord, making it one of the largest military bases in the country.

The base was home to an emotional homecoming earlier this month as a group of 170 soldiers returned from Iraq. Another 100 soldiers are slated to return to the base in the coming weeks.