Truck driver Jerry Gammon told officials he was making fuel deliveries Friday, heading south on the Capital Beltway near Largo, when the front end of his propane-loaded tanker began to shake violently. He lost control and the tanker flipped over, skidded 50 feet and came to rest against a guardrail, officials said. Two small propane tanks exploded, sending metal fragments into the air and sparking a fire that would threaten to erupt in a huge, fiery explosion. The main 2,400-gallon tank did not explode, but the accident caused a major traffic and logistical headache, closing the interstate highway in Prince George's County until 6 a.m. yesterday and causing the evacuation of about 400 persons from nearby homes. "When you consider all the aspects of the situation, the fact there was no more injury, no property damage and no deaths is miraculous," said county Fire Chief Jim Estepp. Gammon escaped without serious injury. Yesterday, federal, state and county officials were trying to determine exactly what caused the potentially disastrous accident. Prince George's fire officials initially suggested that a tire on the truck blew out, but transportation officials said yesterday that they will investigate the possibility of mechanical failure. Officials said no charges have been filed in the accident. National Transporation Safety Board officials will inspect the burned remains of the propane truck, which had been in the shop two months ago for front-end work, according to Charles Batten, chief of the hazardous materials division. He said the board will try to determine why and how seriously the tank ruptured. Because of the threat from the fire, officials ordered all people within 3,000 feet of the site to be evacuated. Traffic was diverted to side streets, causing major backups, and Andrews Air Force Base was advised to keep flights away. Instead of exploding, however, the fuel burned off slowly, creating a "blow torch effect" and taking 13 hours to burn off. During that time, 175 firefighters stood by, unable to get close to the fire because of the threat of explosion. In the meantime, a three-mile stretch of the Beltway, the busiest highway in the area, was closed and more than 30 Maryland state police officers worked the area. The tanker, owned by Poist Oil Co. in Laurel, was righted about 4 a.m., then lifted by crane and towed away. Although no price tag has been affixed to the fire, Estepp said his department probably spent more than $200,000. There is no provision, he said, for charging the oil company. Most of those evacuated went to the homes of friends and relatives, officials said. But about 65 of the displaced, many of them elderly residents of a mobile home park, spent the night in the Largo High School lunchroom, watching TV news of the accident and eating cheese sandwiches prepared by the Red Cross. They began returning home after 3 a.m. "It was a mess," said Alice Hutchinson, 63, who piled into her son's car with six relatives when her neighborhood was evacuated. She spent the night with her sister, returning home at 6 a.m. yesterday. "It really made me mad, but what was I to do, stay here and get blown up?" "It feels wonderful to be back," said 73-year-old Edith Ferguson, who was evacuated from the Fernwood Mobile Home Park. "I was worried and afraid I wouldn't be able to get back. I love this mobile home."