Warning lights on the tower were not working, a TV station official said.
The UH-60 Black Hawk, bound for the Red River Army Depot in Texarkana with seven soldiers from Fort Hood aboard, went down in a field about 30 miles northeast of Fort Hood.
The fog was so thick when emergency crews arrived that they could not see more than halfway up the tower, authorities said.
Everyone aboard was killed, said Lt. Col. Jonathan Withington, spokesman for the 4th Infantry Division, based at Fort Hood. Their names were not immediately released.
Rock Eicke, who lives about a quarter-mile from the crash site, said he was getting ready for work at about 7 a.m. when he was startled by a loud sound. He looked out his window and saw the helicopter hit the ground.
"All of the sudden I just saw a big ball of fire erupt from the ground and then boom, an explosion," said Eicke. "It was burning to the point that we couldn't have done anything."
The main part of the fuselage went down in a field about 200 to 300 yards from the tower, said McClennan County constable Ken Brown. He said he saw the tail section, parts of the rotor and other helicopter parts between the tower and the field.
Base spokesman Dan Hassett did not know what kind of mission the helicopter was on, but said the soldiers were attached to the 4th Infantry Division.
"Our hearts and condolences go out to the family and friends of these soldiers and we're going to do all we can to support the families in their time of grief," Withington said.
The helicopter hit a wire that stabilizes an 1,800-foot television broadcasting tower, said Jerry Pursley, general manager of Waco-Temple-Killeen television station KXXV, which owns the tower. The tower itself was not hit, he said.
He said the station notified the Federal Aviation Administration. The agency's spokesman in Texas did not respond to a call seeking comment.
The Black Hawk, which the military began using in 1979, is the Army's main troop transport helicopter. It can carry 15 people and usually is flown by a crew of four.
In November 2003, 17 soldiers were killed when two Black Hawk helicopters crashed in Iraq, apparently as the result of enemy fire. At the time it was the deadliest single event for U.S. troops in Iraq.