Two explosions rocked a highway construction site early Tuesday, killing six firefighters battling suspicious fires at the scene, officials said. The force of the blasts shattered windows 10 miles away.

A seventh firefighter, who was near the site with a department car, was injured and taken to the hospital, said Fire Department spokesman Harold Knabe.The first blast, shortly after 4 a.m., involved an estimated 30,000 pounds of ammonium nitrate, Knabe said. About 20 minutes later there was a second explosion that involved about 15,000 pounds of ammonium nitrate.

A firetruck was destroyed by the powerful explosions, Knabe said. "There is absolutely nothing to indicate it was even there."

Knabe said a first fire was spotted in a pickup truck while two guards looked for someone they thought they had seen in the area. A second fire broke out in a trailer that contained explosives, and it spread to a second trailer.

"We're assuming that someone did set both fires because both of them did occur when both security people were away from the area," Knabe said. The guards were not hurt, he said.

The blasts left two large craters, 30 to 40 feet wide and 6 or 7 feet deep.

Firefighters and a bomb squad moved into the area of the blast around daybreak to look for the missing firefighters and other explosives.