The Pinal County Sheriff's Department says it will ask for murder charges against two Phoenix residents after a man with heat exhaustion died after being found in a truck with other immigrants.

The man who died was among eight heat-exhaustion victims found Sunday in the back of a truck parked at an Interstate 10 gas station at Picacho Peak northwest of Tucson, the office said Wednesday.

The office was notified early Wednesday that the victim, a man from Mexico, had died at a hospital, sheriff's spokesman Tim Gaffney said. The victim's identity is being withheld pending notification of his family.

The sheriff's office will request a felony murder charge be filed against the driver, Nathan Lamb, 25, and his passenger and girlfriend, Elizabeth Kay, 25, Gaffney said.

They already were being held on multiple counts of charges of human smuggling, aggravated assault and endangerment.

Gaffney said he didn't know whether Lamb and Kay have attorneys but said the court likely will appoint defense lawyers for them.

Information provided by the Border Patrol and those in the truck indicated they had entered the country illegally, Gaffney said.

The people in the truck said they were traveling to Phoenix after being picked up in Douglas by a man who was driving the U-Haul truck and a woman who was the front-seat passenger, the sheriff's office said.

During the ride, many in the group of eight became severely dehydrated and overheated in triple-digit temperatures inside the truck, according to authorities.

Someone called 911 after seeing people stagger out of the vehicle when it stopped.

Lamb and Kay told detectives that they heard the people in the back of the truck pounding and banging from inside the cargo area but were afraid to stop right away, the Sheriff's Office said.

When they did stop and then saw the condition of the people in the truck, Lamb and Kay fled, the Sheriff's Office said.

Sheriff Paul Babeu said previously that the temperature in the back of the truck may have exceeded 180 degrees.

According to the Sheriff's Office, the people being transported each paid $5,000 to be driven to Phoenix.