Army Chief Warrant Officer 3 Dustin Lee Mortenson, 32, of League City, Texas, and Chief Warrant Officer 2 Lucas Maurice Lowe, 33, of Hardin, Texas, died of injuries sustained in the crash.
"The loss of these Guardsmen will be deeply felt across our entire force," said Maj. Gen. John F. Nichols, adjutant general of the Texas National Guard. "Our top priority remains with supporting their families and all investigative efforts. Our thoughts and prayers are with their families during this tragic time."
The soldiers were conducting a "routine training flight" when the helicopter crashed into the water just off the shore of the Bayport Cruise Terminal, about 25 miles southeast of Houston, about 4 p.m. local time, according to a Texas Military Department statement. The Texas Military Department oversees the state's Army and Air National Guard.
Chief Warrant Officer 5 Glen Webb, the senior warrant officer for the Texas Army National Guard, said an investigation into the incident had been launched, but Thursday morning there was no information about what caused the crash.
"Until the investigation is concluded, we won't have answers," Webb said.
Witnesses to the crash told Houston-area television stations that the Apache broke up in the air before it crashed into the water.
Mark Gonzales told Houston's ABC13 that he was fishing in the bay when the Apache approached the coast along a normal military training flight path.
"I could see the helicopter coming," he said. "I heard an explosion, and I saw it crash."
Air and boat crews for the U.S. Coast Guard responded to the crash with local and state fire and law enforcement agencies, according to the Texas Military Department.
The soldiers were assigned to the 1st Battalion, 149th Attack Helicopter Battalion, at Ellington Field Joint Reserve Base near Houston, Webb said.
He said both pilots had extensive experience flying Apaches.
"Our pilots are very competent and they are well-trained," Webb said.
Thursday morning, search and rescue operations were continuing for the scattered remnants of the attack helicopter, an official said. Both pilots' bodies had been located, but it was not clear whether they had been recovered.
Last year, the Army briefly grounded much of its helicopter fleet following fatal crashes during training missions, including an AH-64 Apache that wrecked in South Korea in November 2015. In that incident, the helicopter was reported to have hit a power line or pylon before crashing, killing both pilots aboard.