A piece of metal road debris "the size of a large grapefruit" smashed through the windshield of a minivan on a busy freeway Thursday, killing two passengers, authorities said.

The debris appeared to have struck the van by accident, but investigators have not determined where the object came from, California Highway Patrol (search) spokesman Alex Delgadillo said.

"It was a tragic accident," he said.

He did not know what kind of debris was involved, but said it was "the size of a large grapefruit."

The van was struck shortly before 6 a.m. as it traveled southbound on Interstate 5 in the Atwater Village (search) area, about 5 miles northeast of downtown.

The right front passenger and a man sitting right behind him died of blunt trauma to the head, Delgadillo said. The driver and three other male passengers in the three-row van escaped injury.

Their identities were not immediately released.

Televised news reports showed the van stopped on the freeway median with a large, jagged hole in the passenger side of the windshield that firefighters later covered with a white sheet.

Rush-hour traffic in the area was backed up for miles as police briefly shut down southbound lanes.

Police Chief William Bratton (search) went to the scene because of early concerns that the car may have been the target of gunfire.

On Wednesday, a car-to-car shooting killed a man and injured another on the Harbor Freeway (search) in South Los Angeles. That shooting occurred a few miles away from the site of another shooting on the Harbor Freeway where a driver was killed last month.

No arrests have been made in either case.