The California Highway Patrol is warning motorists along freeways in Riverside and San Bernardino counties to be cautious in the wake of dozens of instances in which someone fired projectiles, possibly pellets or BBs, at moving vehicles, shattering windows and drivers' nerves.

To date, more than 50 vehicles have been hit, mostly along the 10, 15, 91 and 215 freeways, CHP officials said. The result usually has been blown-out back windows. No one is known to have been injured, said CHP Officer Maurice Walker.

Whoever is behind the attacks appears to be firing pellets or BBs, said CHP Officer Scott Beauchene. The attacks, which have occurred during the last several weeks, have targeted many kinds of vehicles at widely dispersed sites and at different times of day, officials said.

The varying pattern makes it unlikely that projectiles have been fired from freeway overpasses, the CHP said.

"We think this is happening from a vehicle," Beauchene said.

The most recent attacks occurred Friday along Interstate 10 just west of Beaumont, Beauchene said. Two vehicles were struck in the eastbound lanes in the morning, and a third was hit in the afternoon in westbound traffic, he said. All were struck in the rear windows. "No one has really seen anything," Beauchene said. "Obviously, somebody is targeting these vehicles on the freeway. I guess you could call it a sniper."

Authorities urged motorists whose vehicles are hit to pull over to a safe spot and call 911. The CHP also asked drivers to note as much information as possible about where the incident occurred and any vehicles they may have seen at the time and report those details to police.