It was so hot inside the car, prosecutors say, that one of the two baby girls dying in a car seat pulled out her blond hair in "clenched little fists."

Sydney Pippin, who was 4 months old, and her cousin, Vicky Fraley, 16 months, died as they sat in the car for eight hours Saturday while the temperature climbed. The girls died of heat exhaustion and dehydration.Ricky Leon Crisp, 23, Vicky's father, and Justin Griffith, 27, are charged with first-degree murder. Authorities said the girls died as the men looked for arrowheads and mushrooms 90 feet from the car.

Crisp and Griffith had been held on charges of manslaughter and endangering the welfare of a child. But prosecutors upgraded the charges Monday, citing a state law that allows a first-degree murder charge for knowingly causing the death of a child 14 or younger.

The men had parked along a dirt road near the tiny northwestern Arkansas community of Little Flock to do their searching near a cave. The windows of the car were rolled up and the temperature outside rose to about 80 degrees, authorities said.

"It appears that the pain was so severe, the trauma so bad, it was so hot in that car, that the 16-month-old - her head hurt so bad - she had her own hair pulled out by the roots in her own clenched little fists," prosecutor Brad Butler said.

"You could safely say within 30 minutes you could be dealing with a 140-degree vehicle," said Dr. Steve Schexnayder, a specialist in pediatric critical care at Arkansas Children's Hospital in Little Rock.

"It doesn't take very long in that heat to overwhelm your body, and children, especially young children, don't have the ability to handle heat or cold as well," he said.

The girls had been with Crisp since Friday night, after the mothers were unable to find child care. The two women were at work Saturday.

When the men found the girls in the car, they started crying and Griffith drove to the hospital while praying and screaming, police said.

"It appears to be an accident, the way he described it to me," said Gary Crisp, who spoke with his son Sunday. "He's kept these babies since they were newborn babies. He's always taken real good care of them and been a real good dad."