PHOENIX —  A pickup truck jammed with as many as 60 people crashed and rolled in a remote area of central Arizona Sunday morning, killing four.

Investigators believe the people on board were illegal immigrants, said Vanessa White, a Pinal County Sheriff's Office spokeswoman.

She says the truck was carrying possibly 50 to 60 people. An estimated 20 to 30 people fled on foot from the accident scene into a desert area and were never located, White said. No arrests have been made at this point, but officers are looking for the driver who fled the scene.

Helicopters flew 18 injured passengers to area hospitals in Phoenix and Tucson and nine were ground transported to Phoenix and Tucson hospitals, White said.

The four who died were all males. The investigation is on going, but it is believed that speed was the ultimate factor in the rollover and alcohol could have been a factor, White said.

The crash occurred around 5:30 a.m. near the town of Arizona City, about 60 miles south of Phoenix.

White did not know whether the pickup was enclosed.