Arizona police, already on alert after a string of freeway shootings, checked out two more possible attacks on Thursday — including a report that a bullet hit a tractor-trailer.

Ten cars and trucks have already been struck, mostly by bullets but also by other projectiles, since Aug. 29, all on or near Interstate 10.

The attacks have raised fears that a sniper — perhaps more than one — is targeting drivers.

On Thursday morning, police said they were checking a report that a projectile damaged a car's window on I-10. Later in the morning, police were looking into a separate report that a bullet struck a semi.