The Washington State Patrol said three vehicles were hit with rocks at about 3:45 a.m.

VIDEO: Several Vehicles Hit By Rocks In Seattle

One of the vehicles hit was a United States Postal Service semi truck. The windshield was shattered by a grapefruit-sized rock, spraying glass inside the cab. Unhurt, the driver was able to drive the truck off the freeway and was towed.

Police said the rock could have killed the driver if it would have gone all the way through the windshield.

SLIDESHOW: Rocks Thrown From Overpass Smash Cars

About the same time, Irwin Buenaventura was driving home from work in the same area when he heard something hit his car.

“I heard something on the hood and I thought it was kind of scary,” said Irwin Buenaventura. He thought he'd run over something, until he got home and saw the damage.

A rock hit the front of Buenaventura's car, scraping and denting the hood.

"It almost hit my windshield. If it hit the windshield, I don't know what's going to happen to me," Buenaventura said.

A third car was also hit. Authorities don't know where the rocks were coming from, but they speculate someone threw them from an overpass. Troopers said throwing rocks from an overpass is a criminal offense.

"(It) could be pranks. It wouldn't be totally unheard of, but not a very funny prank. If we find them they'll be prosecuted to the fullest extent," said Trooper Jason Greer.

Troopers said no one was injured, but all the drivers were shaken by the experience.

State troopers are investigating, but said it’s unlikely they'll be able to figure out who's responsible. However, they did troopers are keeping a closer eye on overpasses and bridges.

Troopers also want drivers to be aware.

“If they see someone in these areas like on an overpass and it doesn't look like they belong there, like they're not walking across, they're just hanging out, let us know so with can be proactive instead of reactive. People will do jail time if convicted,” said Sgt. Keith Trowbridge of WSP.

The incident is the second in which rocks were thrown from an overpass onto cars this week.

Previous Stories: May 27, 2009: I-5 Rock Throwers Say They're Sorry In Jailhouse Interviews