The two-seat OH-58D Kiowa Warrior observation helicopters crashed after 8 p.m. (11 p.m. ET) in the southwest training area of the sprawling base near Tacoma, Wash., according the Army.
KIRO TV reported that local fire crews reached the crash sites, but there were no survivors. The victims were not immediately identified, even by unit, pending notification of relatives.
It was not immediately clear whether the aircraft collided or crashed separately.
"We don't have details on what actually occurred," base spokesman J.C. Mathews said. "That will be part of the investigation."
He was unable to say whether the wreckage of the two helicopters was found in close proximity.
The crash site is geographically closest to the civilian community of Rainier, which is south of Tacoma, Mathews said. There were no injuries on the ground, KCPQ TV reported.
There are more than 40,000 military personnel stationed at Joint Base Lewis-McChord, and with dependent family members the population is 100,000, KCPQ TV said.
Base officials secured the crash site late Monday and immediately began an investigation. The Combat Readiness Center at Fort Rucker, Ala., will lead the overall investigation into the accident, base spokesman Joe Piek said.
"Our thoughts and prayers go out to the family, friends and loved ones of the soldiers involved in this tragic accident," said Maj. Gen. Lloyd Miles, acting senior Army commander at Lewis-McChord and deputy commanding general of I Corps.
"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 this evening," he said.
Temperatures at the base were around the mid-20 degrees Fahrenheit on Monday evening, and local media reported a likelihood of fog in the woods where the crash was said to have occurred.