The deaths of six firefighters killed in two massive chemical explosions sparked by a suspicious fire are being investigated as homicides, police said Wednesday.

A special Police Department squad of six homicide detectives and one arson detective looked into the deaths and the cause of the fire that led to the blasts early Tuesday at a south Kansas City construction site.Police questioned four people Tuesday night to "evaluate their alibis" but no arrests have been made, Police Department spokesman Gregory Mills said Wednesday.

The first explosion occurred while the six firefighters battled a blaze of a construction trailer containing 30,000 pounds of ammonium nitrate with a kerosene mix. That explosion spread fire to a second trailer containing 15,000 pounds of the same highly flammable chemical, leading to a second blast.

The firefighters had spotted the trailer blaze while extinguishing a pickup truck fire about a half-mile away on the same construction site. Two security guards found the vehicle on fire while searching for intruders seen earlier at the site.

The blasts obliterated the trailers, left two craters about 35 feet wide and 7 feet deep and sent vibrations throughout the metropolitan area. One of the two fire trucks that had been called to the scene was left as nothing more than scraps of metal strewn across the site, while the second truck was in pieces but still recognizable.

The suspicious activity reported by the guards led to the arson investigation, Police Chief Larry Joiner told the City Council in an emergency noon meeting.

Ammonium nitrate was blamed for one of this country's worst disasters, the explosion of a shipload of the chemical in 1947 at Texas City, Texas, that killed more than 550 people, injured 3,000 and caused more than $500 million damage.