The two-seat reconnaissance choppers crashed after 8 p.m. in the southwest training area of the sprawling base, according to the Army.
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."
The OH-58D Kiowa Warrior helicopters were on a training mission, Mathews said, further details will be part of the investigation.
The Kiowa Warrior, made by Textron Inc.'s Bell Helicopter unit, is a single-engine, four-bladed aircraft used for armed reconnaissance, Mathews said. It's often called a scout helicopter.
The crash site is geographically closest to the civilian community of Rainier, which is south of Tacoma, Mathews said.
Early Tuesday morning, two sheriff's vehicles blocked access to a rural plot of land where officials erected large sets of lights to illuminate the crash site.
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."
In December 2006, a UH-60 Black Hawk helicopter from Fort Lewis crashed southeast of Seattle during a night training mission, killing all three aboard.
Joint Base Lewis-McChord is one of the largest bases in the country, with about 100,000 military and civilian personnel.