At the scene, only one twisted chassis remained early today as evidence of two fire trucks that arrived with the six firemen before 4 A.M. The wreckage lay near two craters, 30 to 40 feet wide and about seven feet deep.

Later today, after questioning two security guards who had been at the scene, the police put together the following sequence of events:

The guards, stationed there by the Brown Brothers Excavating Company, which is excavating for a new highway, saw someone moving near the edge of the work site and went to investigate.

A fire then broke out in a pickup truck that the guards had parked nearby. The firefighters arriving for that fire then saw fires burning at two trailers that had been left parked at the site. Both trailers contained large amounts of ammonium nitrate, an explosive used for exavation jobs. Details Are Sketchy

The first of the two explosions occurred about 4:10 A.M., the second about a half-hour later. Investigators did not know how many victims had been killed in each blast.

The site is a sparsely populated area on the southwest edge of the city about a quarter-mile from Bannister Mall, one of the city's biggest and busiest shopping centers.