On the last night of her life, Roslyn Whitehead got into a car with someone who drove her to a scenic overlook along the George Washington Memorial Parkway.

Her bloody body, partially clothed and pocked with stab wounds, was found in the parking lot of the overlook at 5:45 a.m. June 4.

That's about all police know about how Whitehead died.

Their search to learn more has been complicated by the same obstacles that made her life such a mystery. Whitehead, 47, was homeless, mentally ill and out of contact with the world -- her family, the network of shelters and mental health organizations that had tried to help her, even with other homeless people.

"The frustrating thing is, we're basically at a loss," said Capt. Robert Rule, assistant commander of the criminal investigations branch of the U.S. Park Police, which is investigating the slaying because the body was found on federal land. "In any homicide, the best way to proceed is to go back to where the person worked, where she lived, her friends, her associates. When you take that away, it makes it very difficult."

In the weeks since a passing motorist found Whitehead's body and called 911, police have plastered her picture in the areas of Rosslyn where she used to go. They have canvassed homeless shelters and chatted up homeless people, seeking to retrace her tattered steps. They have looked for attacks against other homeless people but have found no pattern.

What has emerged slowly is a portrait of an erratic life that took Whitehead from the streets of Arlington to mental health facilities in New York and New Jersey and back to the D.C. area, where police and people who saw her say she had deteriorated into a haze of drinking and delusion in her final days.

And there are clues that have tantalized. A merchant in Ballston said he thinks he saw Whitehead the day before she died, but police said he might be confusing her with another homeless person.

Perhaps most intriguing are signs that while Whitehead apparently had a severe mental illness, reality intruded occasionally. While living at a mental health facility in Manhattan, she remembered to register to vote. And she was cashing Social Security checks, which she might have obtained because of her disability, at a liquor store in Montgomery County, law enforcement sources said. It is unclear if the checks are related to her death.

"She was not unable to get through life," said one law enforcement source. "Not very well obviously because she was homeless and had other issues, but she was able to know when to pick up the check, travel from place to place. Certainly, she was functional in that way."

Whitehead, who did not have a car and usually kept to herself, also was mentally aware enough to get into a car with someone before her death. Police said the scenic overlook where her body was found, about three miles north of Reagan National Airport, is only accessible by car -- unless someone walked at least a mile on the highway.

"Could she have walked there? Possibly,'' Rule said. "Is that the scenario we think happened? No. The question is, 'Why would she have gotten into a car with someone?' "

Whitehead's body was found lying on the north end of the overlook's parking lot. Her body contained no identification but was identified through fingerprints, compiled from her past brushes with Arlington County police. No weapon has been found, and police said they're not sure what kind of knife was used.

Their painstaking investigation has included everything from checking for violent offenders known to frequent the area to scanning speeding tickets from the night of the slaying. Police are pinning some hope on forensic tests, the results of which are not yet available. Rule said a "substantial amount" of blood was found on Whitehead's body and in the parking lot.

But the body does not reveal the person -- and much about Whitehead remains unclear. Police could not say where she was born, where she grew up or how she became homeless. They have spoken to her sister, who last had contact with her in 2001, but other family members could not be located, despite an extensive search.

What is known is that Whitehead was homeless as early as 1988, when she was arrested for trespassing in a building near the Arlington County courthouse.

Whitehead's trail picks up nearly a decade later in Manhattan, where she spent June 1997 to December 1999 living at a mental health and rehabilitation facility called The Bridge Inc. Executive Director Peter Beitchman said she left unexpectedly and showed up at a facility in Newark.

A Bridge employee who knew Whitehead said she was "very friendly," loved to cook Southern food for other clients and occasionally would be well enough to go to the movies. But when Whitehead went off her medication, this employee said, she would leave for days at a time and "do weird things like beg on the subway."

"When she was on her meds, she was okay," said the employee, who spoke on condition of anonymity because of the sensitive nature of Whitehead's illness.

By July 2000, Whitehead was back in the Washington area and back in trouble. On July 6, Arlington police found her passed out under a bush near the courthouse, with bloodshot eyes and reeking of alcohol, law enforcement sources said. Court records show she was convicted of public drunkenness and fined $50. The court file describes Whitehead as 5-foot-4, 150 pounds, with brown eyes and black hair, and as living at "no fixed address."

Arlington police next encountered Whitehead in March of this year, when she was arrested for public drunkenness twice in four days. Still, said Arlington police spokesman John Ritter, Whitehead was "somewhat of a mystery to us."

"Even for someone who was homeless, she kind of stayed to herself," Ritter said. "She didn't have much contact with other homeless people."

Local organizations that work with the homeless said they barely knew Whitehead, although she did stay at a shelter in Montgomery County in mid-April.

Olga Garcia, an outreach worker for the Arlington Street People's Assistance Network, said she saw Whitehead only once, about six weeks ago, when she was handing out hygiene supplies to the homeless in an Arlington park.

"She came up to me and asked me for a toothbrush," Garcia said. "She had a T-shirt on and looked like she'd been drinking. She just looked tired."

About the same time, Clark Carvaly, who is also homeless, said he encountered Whitehead shuffling down the street in front of the Rosslyn Metro station. "I don't think she spoke at all," said Carvaly, staring at a picture of Whitehead that authorities had taped to a storefront. "She was kind of walking like a drone. She just looked troubled, like she was not really in this world."

Staff researcher Bobbye Pratt contributed to this report.

Roslyn Whitehead suffered from mental illness.