A 3-year-old Sterling girl was killed yesterday morning in Loudoun County when the car in which she was riding with her father and little brother was struck head-on by another car, police said. The three were headed to the children's grandmother's house, where she usually baby-sat for them.

Loudoun County sheriff's deputies said the accident, which also left three people injured, occurred about 8:40 a.m. when a car driven by a Gainesville woman crossed the center line and collided with the station wagon driven by Morgan Caywood, 22.

Caywood's daughter, Kierstan, 3, was riding in the front passenger seat, and was not in a child safety seat or secured by a lap seat belt, sheriff's deputy Doug Taylor said. Kierstan died at the scene, he said. Two-year-old Andrew was riding in a purple child safety seat in the back seat, deputies said. He was treated and released from Loudoun Hospital Center.

Morgan Caywood, who was wearing a seat belt, was taken by helicopter to Inova Fairfax Hospital, where he was listed in fair condition last evening, according to a hospital spokeswoman.

Caywood's father, Craig Caywood, said his son was following his daily routine of driving the children to his mother-in-law's house in Sterling before heading to work at a local telecommunications company. "He's been on that road a million times," said Caywood, adding that police had told him his son never had a chance to react to the impending crash because it had happened so quickly. "He was just driving," he said.

Kierstan "was the most beautiful girl you ever saw," Caywood said, adding that she was an outgoing child who loved to hug her relatives. "She could give you a hug that would break your heart."

Craig Caywood said his son is "pretty beat up" but that doctors think his injuries will not be permanent. His son and daughter-in-law, Madeline, are expecting their third child, he said.

Investigators said Tonya Chunick, 19, was driving a 1992 Honda on Old Ox Road and veered off the right side of the road near the western entrance to Mercure Circle. She then reentered the road and crossed the center line before hitting the Caywoods' car.

Taylor said investigators do not know why Chunick lost control.

"She runs off the right, she over-corrects and ends up going into the westbound lanes," Taylor said. He said the investigation is continuing and no charges have been filed.

Chunick was taken to Washington Hospital Center, where she was treated and released.