Rocks, some the size of soccer balls, pelted houses and cars on a Loudoun County cul de sac Thursday afternoon in a bizarre blasting accident as land was being cleared nearby for development, residents and officials said.

At one house in the Broadlands subdivision near Dulles International Airport, the rocks ripped holes through the roof and smashed through kitchen windows and the back and side walls. At another, a stone dented the garage roof and thudded onto a flower pot, splitting it in half. And at a third house, a hefty hunk of basalt dug a small crater in a mulched flower bed before coming to rest next to a stand of pink and white petunias.

County fire officials said the accident is under investigation. A crew was blasting diabase rock on a nearby tract of land, clearing the way for the basement of a new home, when an eight-foot-thick boulder split and rocks under it flew into the air, the developer said.

"This instance, it's kind of like a freak situation, like lightening striking," said Thomas J. D'Alesandro IV, a regional vice present for Terrabrook, the developer of Broadlands. "You have a strange rock formation that's very unusual."

Such may be the dangers of living at the edge of development in the third-fastest growing county in the country.

Residents said they're used to the dump truck traffic. They've grown accustomed to their houses shaking and rattling as developers blast away earth to make way for new construction. And they're even used to the noise of hydraulic jackhammers.

But nobody was prepared for flying rocks.

"It's just scary to think you could be sitting in your house and a boulder could fly through your window," said Emily Broad, 31, whose white Saturn station wagon was scratched by rocks on the hood and passenger side.

Broad was home at the time, talking on the phone. She heard three horns, a warning that blasting is about to begin. Then the rocks rained on the roof. She went outside later and saw so many rocks on her lawn that she thought her son had been collecting them.

Neighbor Allen Sackadorf, 35, who wound up with a honeydew melon-size rock in his yard, said he was worried for his family's safety. "It's nuts," Sackadorf said. "It really makes you question if these guys know what they're doing. . . . I have three kids. Every house in here has kids."

The blasting was done by William A. Hazel Co., which completes site work in preparation for development. They were blasting a little more than 200 feet from the nearest house.

Gerald Naquin, safety director for Hazel, said the company had taken standard precautions and is still trying to figure out exactly what went wrong.

"It's not something we intentionally do," Naquin said, adding that Hazel would pay to repair damage. "I'm a homeowner also. I can understand their concerns. I can understand their anger and their fears."

D'Alesandro said blasting in the area had been suspended until the cause of the accident is determined.

The damage ranged from an outdoor brass light that was scratched and knocked on its side to a house that looked as if someone had lobbed a few baseballs though it.

At one house, rocks pierced the shingles above a baby's room, causing the basement wall to buckle, breaking kitchen windows and knocking diplomas off the wall on the first floor, said Broad, who saw the damage.

Nearby, at the home of Sheila and Brad Ryan, where a rock dented the garage roof, the side view mirror on Brad Ryan's new Ford Contour was shattered and bent.

And then there was the boulder that found its way onto Michael R. Hieb's lawn. It bounced once before coming to rest in the mulch.

After picking up the rock, Hieb concluded: "It weighs enough to kill someone, that's for sure."

CAPTION: Michael R. Hieb holds a rock that fell into a flower bed in his yard in Loudoun on Thursday. Officials say the rock, and others, hit the neighborhood while workers were using explosives to clear land for a nearby development.