Police on Sunday were still trying to determine who was driving a stolen truck loaded with illegal immigrants that smashed into other vehicles and rolled over, killing six people and injuring 15, some critically.

Immigration and Customs Enforcement (search) officials questioned a man they had believed was the driver, but he turned out to be a passenger, said Arizona Department of Public Safety (search) Sgt. Brian Preston.

Investigators think the pickup truck was traveling at about 90 mph when it lost control at an intersection near Fort Huachuca, an Army post next to Sierra Vista (search) in southeastern Arizona, Sierra Vista Deputy Fire Chief Randy Redmond said Sunday.

Seventeen people were in the truck, including two women, 14 men and a boy, Preston said.

Deputies had been chasing the truck, but broke off pursuit before the accident.

Five people died at the scene of the 11-car crash Saturday and one victim died later a hospital. Of the 15 injured, one person was on life support and six were in critical but stable condition, Preston said.

"It was a pretty horrific wreck," he said.

Cochise County deputies had started trailing the truck because they believed it was carrying illegal immigrants, sheriff's spokeswoman Carol Capas said. Officials said it was not immediately clear where the truck had come from.

Capas said the deputies called for backup and stopped chasing the truck when the suspect drove away recklessly.

The driver lost control after swerving at a spot where police had placed tire deflation spikes to slow the truck.

"Speculation was that maybe he thought he would drive around them," Preston said.

The passengers were thrown out as the truck struck a center median and hit vehicles waiting in a turn lane, Redmond said.

The four-door pickup did not have a camper shell or cover.

The impact tore the roofs from some cars and trapped several motorists in their vehicles, Redmond said.

James Lee, 74, and Emilia Lee, 71, of Huachuca City, were killed when the pickup hit their vehicle. The other four people who died were in the pickup; their names had not been released Sunday.

Four suspected illegal immigrants were being interviewed by Immigration and Customs Enforcement officials in Naco, Border Patrol spokesman Andy Adame said Sunday.