An empty fuel tank was the apparent cause of Thursday night's crash of a single-engine plane that critically injured WTOP-radio traffic reporter Steve Thompson and its pilot, according to federal investigators.

A spokesman for the National Transportation Safety Board said yesterday that a preliminary inspection shortly after the crash indicated the plane's fuel tank was empty. That fact, plus accounts from witnesses that the plane's engine was off at the time of the crash, have led investigators to the preliminary conclusion that the aircraft ran out of fuel, the spokesman said.

Charles J. Colgan of Colgan Airways, owner of the single-engine Cessna, said company records indicated the plane's 43-gallon fuel tank had last been filled after a rush-hour flight Wednesday night. Colgan, a Democratic state senator from Manassas, said the plane had been used for both the Thursday morning rush-hour report and again that evening without being refilled.

"There should have been sufficient fuel to fly both trips but we generally refuel for each flight," said Colgan, who said Thompson and Bernard (Wick) Wicker, the pilot, would have checked the fuel gauge before takeoff. "I don't know if they thought they had enough to make it, or if the guage wasn't working or if there was another problem."

Thompson, 34, remained in critical condition at Fairfax Hospital yesterday but was "holding his own," according to hospital spokeswoman Kathy Zajdel, who said, "His chances look better than they did yesterday."

A piece of the plane's steering column was embedded in Thompson's abdomen and he spent five hours in surgery late Thursday night. He was conscious during the day and able to speak to his wife Nancy, according to WTOP-radio news director Armand Asselin.

Wicker, 52, was listed in serious but stable condition with internal injuries. Asselin said he was told that Wicker, who had had tubes inserted into his throat to help him breathe and could not speak, took a pencil and paper and wrote the word "Steve?" when he regained consciousness after surgery Thursday night.

Both men remained in the hospital's intensive care unit last night.

Thompson was in the midst of his evening traffic report Thursday when he radioed the station that the plane was having engine trouble and would attempt to return to Manassas Airport, from which it had taken off about two hours earlier.

Thompson called back about a minute later to say the engine had quit and that Wicker would attempt an emergency landing in Vienna.

The aircraft narrowly missed an unoccupied house, struck a tree, flipped over and came to rest against a second tree. It took rescue workers nearly an hour to cut Wicker out of the wreckage and 90 minutes to extricate Thompson.

An Alexandria resident who is married but has no children, Thompson has been WTOP's traffic-watch reporter for two years. Wicker, who lives in Gainesville, Va., with his wife and three children, is the senior pilot for Colgan, the Manassas company from which WTOP leased the Cessna and another plane.

WTOP's Asselin, who said he had complained repeatedly to Colgan about the maintenance of the two planes based on complaints from Thompson, said yesterday the station would not use Colgan aircraft again until the federal investigation into the crash was complete.

Asselin said WTOP would attempt over the weekend to line up another company to provide aircraft temporarily for the two daily rush hour reports. Yesterday the station relied on ground monitoring of traffic conditions for its reports.

Colgan said he believed maintenance problems did not contribute to the accident and that the company operates under strict federal safety regulations.

Federal investigators said they planned to pull the plane's engine and take it to a Hagerstown hangar for a detailed tear-down to make a final determination of the cause of the crash.