A fatal fire at a gasoline storage depot in Fairfax City last June was ror in loading a cargo truck and by the nonfunctioning of a safety mechanism intended to catch such errors, according to a report by the U.S. Department of Transportation.

The early morning fire at the largest storage terminal in the Washington area was "caused by the driver of the Amoco cargo tank overfilling his cargo tank. . .(and). . .by the failure or lack of an operative sensor system," according to the report by the department's Bureau of Motor Carrier Safety.

"If the liquid level sensor detection system had been functioning properly, it would have shut off the loading pump before the compartment overfilled," the report concluded.

The driver of the Amco cargo tank truck, Newton B. Leaphart, 43, of College Park, died five days after the fire from burns over 80 per cent of his body. Six other drivers received moderate to minor injuries, according to the report.

Leaphart was engulfed in flames when fumes from the gasoline spill were ignited by the backfiring of the engine of another truck. The second truck had pulled up to an adjacent loading position, the report said.

Located on Pickett Road in the heart of a residential area, the tank farm, where the accident occured was built over the opposition of local residents who feared it was dangerous. The facility stores gasoline and fuel oil pumped from the Colonial Pipeline, the 36-inch 1,600-mile underground pipe that carries petroleum fuel from the Gulf of Mexico to New York City.

Both U.S. and state occupational safety officials are investigating the incident to determine whether there were violations of safety regulations. Both departments declined to speculate on the outcome of their investigations.

Fairfax County Assistant Commonwealth's Attorney Randall M. Starrett said that his office also is investigating the incident to determine if any criminal negligence occured. He said experts he consulted to help pinpoint the exact cause of the accident were not in agreement and that all his information has not yet been analyzed.

"It is hard to determine (exactly what went wrong) from a pile of ashes . . . the whole situation does not give itself to easy answers," Starrett said.

The Department of Transportation report relates that two drivers told county fire investigators that the liquid level sensor system had been inoperative most of the time since last January and that it was not operating at the time of the incident. They also said that Leaphart had been involved in another overfill the previous week.

Amoco officials denied that the sensor system was inoperative and said that it had been turned on the morning of the fatal fire. Earl R. Wood, Amoco's terminal manager denied any knowledge of the alleged previous overfill, the report stated.

County fire marshal Lt. Richard L. Stone said that both the county and state safety code require that the sensor system be operative. Fire investigators must see that the system is working properly on their annual inspection tours of the tank farm, but they "are not required to check the internal workings of the sensor system. It's a complicated system that needs someone who's an engineer (to check it out)," Stone said.

The last time the Amoco facilities were inspected by the county fire marshal's office prior to the accident was in November of last year, Stone said.

As a result of the fire, which was the first serious blaze at the 13-year-old tank farm, Fairfax City officials have asked the four oil companies that share the facility, Amoco, Gulf, Texaco and Cities Service (Citgo), to install a heat-sensitive foam fire extinguishing system on their loading racks.

Amoco had planned to install such a system on its loading rack by the end of this year, according to the Department of Transportation report.

Fairfax City's Assistant Manager Robert Norris said that Citgo and Gulf have agreed to install such a system, which costs about $150.000 each, in 1978. The city has received "no word yet" from Texaco, Norris said.