A 10-year-old Arizona boy was killed Sunday afternoon when a sand cave that he and his cousin were digging collapsed onto him, authorities said.

The two boys were tunneling into the side of a sand dune under a 40-foot cypress tree at a crowded Salmon Creek State Beach around 3:30 p.m., when the sand caved in, fire officials said.

"The tree did fall over, but it is purely speculation if the wind or age or tunneling brought the tree down," Bodega Bay Fire Chief Sean Grinnell said.

The victim's cousin was partially buried but was able to dig himself out, and he ran for help, Grinnell said.

It took rescue personnel more than an hour to pull the 10-year-old out of six to eight feet of sand, and they were unable to resuscitate him. His identity was not immediately released.

"It is a very freak accident," Grinnell said.

The victim's family had rented a house in the Bodega Bay area for a vacation, and his parents were nearby when the collapse occurred, authorities said.