A Puerto Rican-born Arlington woman who vanished last Saturday after setting out from home for her job as a chemist at the Quantico Marine Base has become the subject of a widespead search by police in the Washington area, authorities said yesterday.

Maryland State Police said a car belonging to the woman, Bilmaris Rivera, 21, was found ablaze on a deserted dirt road near Frederick on Sunday, but that there was no trace of the owner. Police said the fire was deliberately set.

Arlington police spokesman Tom Bell said yesterday Rivera currently is listed as a missing person, "but an awfully suspicious missing person (case). It doesn't look good at this time." Police have no suspects, Bell said.

Rivera, who moved to this area from Puerto Rico last November, was described yesterday by a family member and friends as a studious woman who wanted only to be admitted to Georgetown Medical School.

"I never knew her to be out late or anything like that," said Maria Irizarry, who shares a tiny apartment at Rosslyn's Arlington Towers Apartment complex with Rivera and another friend. "She used to study all the time. She was the kind that was home by 9 at the latest."

Juan Rivera, the woman's father, flew to Washington from his home near San Juan to be on hand as police hunted for his daughter. "I hope in God," he said, when asked if he believed his daughter would be found alive. "That's what I'm praying for."

Police theorized yesterday that Rivera may have been abducted by someone who stopped to offer help when her car broke down. The car, a green 1974 Ford Pinto hatchback, reportedly had a faulty transmission and Irizarry said Rivera had planned to have it repaired this week.

According to Arlington Police Sgt. Clyde Hall, the car was found enveloped in flames early Sunday morning in a wooded area of Montgomery County. He said the vehicle had been doused inside and out with a flammable liquid and deliberately set afire. The car's gas cap and license plates had been removed.

"Right now, we're baffled," said Hall. "This is a good girl, a good planner and organizer. She wouldn't ordinarily pick up hitchhikers or anything like that. But we've got to look at the possibility it's a homicide."

Rivera was described as about 5 feet tall, slender, with brown shoulder-length hair and large-lensed, brown-framed eyeglasses. She was last seen wearing blue jeans and a T-shirt.

Coworkers at Quantico Marine Base, where Rivera had worked for seven months testing water samples in the water plant, described Rivera as a pleasant, young woman. "She was cheerful and she was considered a promising young professional," said Marine public affairs officer David Simon.

Iriarry, waiting in her apartment yesterday for word of her friend's fate, said she last had seen Rivera at about 6:30 a.m. Saturday when her roommate gently shook her awake.

"Wake up, lazy," she said she remembered Rivera chiding her. "Make sure you get up on time."

Rivera then trotted out the door, but did not appear for work at the Quantico water plant. A coworker assumed the woman's car had broken down and did not call police. When Rivera did not return to her apartment that evening, her roommates began calling all her friends.

Nervously fingering a snapshot of her yesterday, the elder Rivera said he had always warned his daughter to be wary of strangers. "In Puerto Rico, too, I told her to be careful. Many places it can happen, no matter if you be careful."