Authorities arrested two brothers today in the slaying of a 73-year-old woman from the Eastern Shore who was struck by gunfire from a pickup truck that had stalked the car in which she was riding for more than 20 miles, as she was returning home from a Christmas shopping trip.

Officials said they did not have a motive for the crime, which they described as a seemingly random attack along a rural Kent County highway. But the FBI confirmed that it has opened a civil rights investigation into the case and is trying to determine if the shooting was racially motivated. The woman was black. The suspects are both white.

David Wayne Starkey, 24, and Daniel Robert Starkey, 19, were arrested before dawn today in Millington, Md., a small town near the Delaware state line. Each man was charged with one count of first-degree murder, two counts of attempted murder, three counts of reckless endangerment and three counts of first-degree assault. They were being held tonight without bond in the Kent County jail.

"I just don't know what would move a human heart to do something like this," said Robert Strong Jr., the Kent County state's attorney. "What would motivate anyone to do such a horrible crime? If I had the answers, I'd be a very wise man."

Kent County Sheriff John F. Price said the brothers were riding in a Chevrolet pickup truck on Route 291 in Millington about 6:30 p.m. Saturday, when for unknown reasons they began to tailgate a subcompact 1988 Plymouth Horizon.

At the wheel of the Plymouth was Michelle Y. Wilson, 38, who had just spent the day shopping in Dover, Del., with her two passengers: her mother, Meriam G. Spriggs, 67, and her mother's cousin, Germaine P. Clarkston, 73.

The driver of the pickup truck flashed its headlights, honked its horn repeatedly and tried to pass and ride up on the bumper of the car, officials said. After a few minutes, the truck slowed down, and the shoppers thought the incident was over.

The truck, however, continued to follow the Plymouth from a distance for about 20 miles until the two vehicles reached the neighborhood where the three women lived, officials said.

As the Plymouth pulled within a half-mile of Spriggs's house, officials said, the truck suddenly raced forward, and a passenger inside opened fire with a shotgun, striking Clarkston in the hip and wounding her fatally. She died Monday afternoon at a Baltimore hospital. Spriggs and Wilson were slightly injured in the attack by flying glass.

"All of a sudden, he pulled up on our left and boom, boom, boom," Wilson, of nearby Chestertown, recalled today. "At that point, all I could do was stop."

Wilson looked down and saw a hole in her leg, then she looked over at Clarkston and saw she had been struck.

"I've been crying every night thinking about Miss Germaine," Wilson said. "They took an innocent life, as far as I can tell, because I wasn't driving fast enough."

Clarkston and Spriggs lived on the same street in Georgetown, a close-knit, predominantly black community in western Kent County. Even with the arrest of the Starkey brothers, the victim's family members said they are still at a loss to come up with an explanation.

"I'm still numb," said Clarkston's son, Linnwood, 55, staring at the dining room table where, on Thanksgiving, he and his mother shared what would be their last family dinner. "I just remember the picture of her in that car and that wound. I'm just sad this had to happen to my mom."

A posse of sheriff's deputies, Maryland State Police and an FBI agent arrested Daniel Starkey about 5:30 this morning at his home in Millington, Price said. Law enforcement officials arrested David Starkey nearby, Price said.

Price said he was going to leave it to the FBI to determine if the shooting was racially motivated. "We have nothing in our investigation to indicate that," Price said.

Price said a tip from a "concerned citizen" led officials to the Starkey brothers but otherwise released few details. He volunteered that "drugs have been ruled out in this case." When asked if the two brothers had been drinking, he replied, "I can't comment on that."

FBI spokesman Peter A. Gulotta Jr. said the results of the agency's civil rights investigation will be turned over to Justice Department officials, who will decide whether to file federal charges.

Kent County court records show that Daniel Starkey pleaded guilty July 8 to marijuana possession. He was sentenced to 18 month's probation and ordered to enroll in a drug treatment program, records show.

Women at the brothers' homes were asked to comment and declined.