Two men who prosecutors tentatively identified as U.S. citizens were shot to death in their vehicle early Monday as they waited at a Tijuana-area border crossing to enter the United States.

Prosecutors in Baja California state quoted witnesses as saying a gunman approached the line of vehicles waiting at the San Ysidro border crossing and fired into the men's pickup truck, hitting the victims in the head, arms and body.

The state Attorney General's Office initially said the men were U.S. citizens aged 25 and 28.

Attorney General Rommel Moreno later said their nationality was still unclear.

"We are looking at where they came from, their nationality, if they are dual Mexican-American (citizens). We do not have that information yet," Moreno said.

The U.S. consulate in Tijuana did not immediately return phone calls seeking confirmation of the men's nationality. There was no immediate information on their hometowns.

Investigators said they found 9-mm shell casings at the scene. That ammunition is used in weapons favored by drug cartel gunmen in Mexico.

The shooting occurred before dawn Monday. The men's pickup had California plates. Both victims were dead by the time authorities arrived.