KANSAS CITY, Mo. (AP) _ A devastating chemical explosion that killed six firefighters at a highway construction site and shattered windows 10 miles away was caused by arson, police said.

Detectives questioned four people to ''evaluate their alibis'' in Tuersday's pre-dawn fire that triggered the blast, but no arrests have been made, said Sgt. Gregory Mills, a police spokesman.

Fire bells tolled throughout the city for the six firefighters, who were killed when a construction trailer loaded with 30,000 pounds of ammonium nitrate exploded, ripping a large crater in the ground and virtually obliterating a fire truck.

Another explosion occurred 40 minutes later in a nearby trailer that stored 15,000 pounds of the material, which is used in highway construction projects to make dynamite explode more evenly.

''A lot of guys just find it hard to believe,'' said fire Capt. Owen Gilchrist. ''No one wants to. Another five or 10 minutes, there could have been 20 or 30 guys down there.''

Mills said investigators determined arson from the presence of two separate fires burning almost simultaneously at the site. One blaze, in a small pickup truck, started in the cab. The second began 500 feet away in the trailer containing the explosives, he said.

Police said they had no motive for the arson, but did not rule out that the fires could have been set as a prank. A special squad of six homicide detectives and one arson detective was working on the case.

Labor union officials said there had been no problems at the site, where workers had been blasting to clear rocks for a highway project. The only reported crime had been a vandalized bulldozer last month, police said.

Two pumpers manned by the six firefighters had been called to the site by security guards who noticed two prowlers in the area and saw the pickup on fire, said fire department spokesman Harold Knabe.

According to fire department transcripts, a dispatcher informed firefighters that explosives were in the area as they approached the scene. Battalion chief Marion Germann had just pulled up in a car about a quarter mile from the scene and was ''in the process of ordering his firefighters out when the explosion hit,'' Mills said.

''He saw a strange burning pattern,'' the police spokesman said.

Germann estimated the firefighters killed in the blast were about 30 to 40 feet from the burning trailer.

The blasts left two large craters, 30 to 40 feet wide and 6 or 7 feet deep, and a smaller one about 15 feet across and 4 feet deep. Thousands of windows were blown out of businesses, homes, schools, banks and hospitals up to 10 miles away.

''The remains of one fire truck is sitting very close to one crater,'' Knabe said. ''There is another vehicle of some sort, a large red truck. The other fire truck - there is absolutely no evidence that it was ever there.''

One witness a few miles away, Cindy Denning, said that when the flash of light brightened her bedroom, she thought nuclear war had begun.

''I woke up and my first reaction was it's nuclear war, or else a gas explosion in my building. I thought the building probably was on fire,'' she said.

Apartment buildings, including Ms. Denning's, and nearby houses were evacuated for several hours by authorities who feared more explosions, Mills said.Debris was scattered over several acres around the construction site and onto nearby U.S. 71.

The dead firefighters were identified as Capt. Gerald C. Halloran, 57; Capt. James Kilventon Jr., 54; firefighters Thomas M. Fry, 41, Luther E. Hurd, 31, and Michael R. Oldham, 32; and Robert D. McKarnin, 42, a fire apparatus operator.

The accident was the worst in the fire department's history. Five firefighters were killed in a 1959 gasoline storage tank explosion.