By his own admission, Josiah M. Miller, 27, of Arcata, Calif., was among the last people to see Michelle Ashlee Dickson, of Crescent City, Calif., before her disappearance the night of July 15.
Miller has told police that he met her about 10:30 p.m. that night on U.S. Highway 101 to purchase three ounces of pot from her, Det. Sgt. Steve Morris, a spokesman for the Del Norte County Sheriff's Office, told ABCNews.com.
Dickson's car was found engulfed in flames off Highway 101 about 8 a.m. the next day.
Police have publicly said that Miller is now a suspect in their investigation into Dickson's disappearance.
"I'm appealing to this young man," Morris said. "We've got a lot of big question marks in his timeline and we're working to nail it down."
Police said Miller described himself as a friend of Dickson but denied knowing where she is. But after initially talking to police, he has invoked his right to an attorney. Attempts by ABCNews.com to reach Miller were unsuccessful.
Suspicion of Miller's possible role heightened when a police search of his truck found what a preliminary test identified as a protein sample. Investigators are waiting for test results to come back from a crime lab.
"We've done some chemical tests that indicate there was protein on the seat," Morris said. "We've sent it off to make sure it's blood." If it is blood, authorities must then determine whether it came from Miller, Dickson or someone else.
Investigators for the Del Norte County Sheriff's Office collected the protein sample after obtaining a search warrant to search Miller's vehicle. The truck, Morris said, had been cleaned and stripped of some of its seating upholstery before the search. The evidence was collected from the foam lining beneath. Authorities also seized Miller's computer and some clothing.
The search for Dickson began last Wednesday when a motorist on U.S. Highway 101 spotted flames coming from a turnout area in Crescent City, a coastal community tucked into Northern California's Redwood forests about 25 miles south of the Oregon border.
The vehicle, Dickson's 2003 hatchback Honda Civic, was about 40 yards off the highway on the ocean side, Morris said. The Del Norte County Sheriff's Search and Rescue Team, armed with cadaver dogs, boats and helicopters, has searched the area four times in the last week without finding any sign of the missing woman.
Miller acknowledged to police that he met Dickson the night before her burning car was recovered to make the drug buy. According to Del Norte County investigators, Miller told police that he grew paranoid about having the marijuana in his truck after he left Dickson and pitched it into a river.
Investigators have not yet confirmed reports placing Miller in Crescent City after 10:30 p.m. the night of the exchange.
Dickson's family has pleaded for any information that might help find their daughter.
"My daughter turned 24 on Saturday," Sandra Davis, Dickson's mother, said in an interview with ABCNews.com. "What do you say about your child? She was a beautiful woman."
Davis, intermittently in tears, described her daughter as a community college student who lived at home and worked a pair of waitressing jobs. She had a pitbull puppy that she would not just leave behind, Davis said, and loved her car. "It was her baby."
Davis knew nothing about her daughter selling drugs and said that she is fearful the marijuana deal could somehow cast her daughter in the wrong light. "I don't want people to focus on the fact that she possibly made a stupid mistake," Davis said. "It doesn't lessen the fact that someone may have hurt her."
Dickson is described as 5 feet 4 inches tall, weighing 120 pounds with brown hair and brown eyes. She was last seen wearing a sweatshirt, sweatpants and sandals.
Morris said that they want to pin down the exact relationship between the missing woman and the suspect. He also said that investigators have looked into the missing woman's background and have been unable to find any evidence that she was involved in drugs beyond marijuana, or other potentially dangerous lifestyles.
"Like all human beings, she had a little bit of a dark side," Morris said. "I think this was her other side. She was trying to make a little bit of gas money by selling a little bit of marijuana."