The two-seat reconnaissance choppers crashed after 8 p.m. Monday in the southwest training area of the sprawling base, according to the Army.
Chief Warrant Officer 5 James Oliphant, an aviation safety officer at the base, said the airmen were on a training flight, but he did not know specifically what they were doing before the crash.
"The actual events are unknown at his point," Oliphant said. He said the investigation could take several months.
Lt. Col. Gary Dangerfield said Tuesday morning that the victims' remains were taken to Madigan Army Medical Center, and officials were still notifying their families. The victims' names will not be made public until 24 hours after family notification.
Dangerfield said the crash occurred during "routine night training."
"One loss is one too many," he said. "Any time you lose a soldier, it's like losing a brother or a sister. It hurts."
The aircraft involved were OH-58D Kiowa Warrior helicopters, often called scout helicopters. The single-engine, four-bladed aircraft are used for armed reconnaissance, base spokesman J.C. Mathews said.
The crash site is geographically closest to the civilian community of Rainier, which is south of Tacoma, Mathews said.
Early Tuesday, two sheriff's vehicles blocked access to a rural plot of land where officials erected large sets of lights to illuminate the crash site.
Oliphant said daylight was allowing officials to get a fuller picture of the debris field, the majority of which was within a 300-meter area.
"With moving aircraft, there could be debris anywhere throughout the woods," he said. Additional military vehicles arrived to help secure the scene.
Oliphant said the weather was clear when he arrived at the crash site Monday night.
Base officials secured the area 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.
Dangerfield said investigators from the readiness center were expected at the scene by Tuesday night.
"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.