Investigators in Provo, Utah, now believe it was missing Brigham Young University senior Camille Cleverley who used her own debit card to buy juice and donuts at a convenience store last Friday, a day after she disappeared, police told FOX News.

Detectives have interviewed the store clerk who was at the cash register at about 11 a.m. Aug. 31, when the transaction was conducted, and the clerk is relatively sure it was 22-year-old Cleverley who bought the items. The Provo shop's security camera wasn't working when the cashier swiped the card on the purchase.

The store is close to her off-campus apartment.

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A passerby came forward this week to report that Cleverley was riding her bike between 5 p.m. and 6 p.m. Thursday afternoon near the BYU library, according to The Salt Lake City Tribune.

If the bike rider was, in fact, Cleverley, that could have been the last known sighting of the student before she vanished. Her roommate saw her at noon that same day.

A man and a woman have been interviewed by police in connection with Cleverley's disappearance and have agreed to take a polygraph, KUTV reported. Provo police also have been investigating a car wash called The Clean Getaway.

The Boise, Idaho, native — who was going into her senior year at Brigham Young and also worked at the university — was reported missing on Thursday by her roommates. No one close to her has heard from her since. She last was seen in the Provo area, where campus is located.

Cleverley doesn't have a history of running away, though she has had some unspecified health issues, FOX News has learned.

Her boyfriend, Dave Sperry, told FOX he last saw his girlfriend Wednesday evening and nothing seemed amiss.

"Everything was good. She was happy, smiling," he said. "Everything seemed to be going well."

Her family has said her white and lime-green striped sneakers are missing from her closet, as are her green basketball shorts.

Investigators have been on the lookout for Cleverley's purple and silver Schwinn bicycle, which is also gone. She didn't have her cell phone with her, police told FOX, but apparently was carrying her wallet and keys.

"Her bicycle is missing, so we assume that she had taken off on a bike ride," her mother, Susan Cleverley, told FOX News. "She had been visiting with us the previous weekend and had left her cell phone at her sister's."

More than 20 people who know Cleverley have been interviewed in the investigation, according to Provo Police Capt. Cliff Argyle.

Meanwhile, searchers embarked on another hunt in the region Wednesday for clues in the case. They scanned the areas bordering Canyon View Park's bike trails, on which Cleverley was known to ride. Some of Wednesday's searchers are from the organization Destiny's Search, which worked on both the Elizabeth Smart and Lori Hacking cases.

The terrain they targeted is dense, and the area was heavily trafficked during the Labor Day holiday weekend.

On Tuesday afternoon, a fixed-wing surveillance plane flew over Rock Canyon, where Cleverley was known to spend time, but found nothing. A group of volunteers with search and rescue dogs also scoured the canyon.

In addition to searching the bike trails, investigators have combed through Cleverley's apartment and other areas around campus where she worked and was known to go, he said.

The chief of police cautioned against drawing any hasty conclusions, saying detectives hadn't focused on any one theory as to what may have befallen the student.

"Right now I'm saying we just don't know" what happened to Camille, Provo Police Chief Craig Geslison told FOXNews.com on Tuesday. "We really don't have anything specific. ... We're just looking at everything at this point in time."

Camille would have started classes at Brigham on Tuesday. The university sent out an e-mail alert to students and faculty Tuesday informing them of the disappearance.

"BYU is obviously very concerned and hopeful," spokesman Michael Smart said. "We are taking our lead from Provo police as to how we can support their search efforts. Students have been involved passing out fliers."

Cleverley's mother said Wednesday that her daughter's messages since the disappearance have been listened to, but they were all just from people expressing their concern.

The young woman's boyfriend has been very cooperative in the investigation, according to police and her family.

"He's been very supportive," her mother said.

She added that the family wasn't aware of any reason her daughter might have run off or sought some time alone. Geslison echoed those sentiments, saying he didn't know of any emotional or other problems Camille was having that would cause her to leave suddenly, without a trace, the way she did.

"We're just looking at everything at this point in time," he reiterated.

Camille's family has also been helping in the investigation, and they were the ones who notified police of her disappearance — after her roommates apparently contacted them when she didn't come home, according to the chief.

As far as whether foul play might be involved, the missing student's mother and police both said they didn't have enough information to go on.

"We have no idea," Susan Cleverley said. Police "have several different scenarios that they’re working on. Right now, we’re trying to get a lead on any one of them."

Cleverley lived in an off-campus apartment and had just met her new roommate before she disappeared, her brother David Cleverley said.

"It could be anything from her getting hurt and lost to an abduction. It's been tough," said 24-year-old David Cleverley, a student at Utah State University in Logan. "It's frustrating to try to put it all together because we have nowhere to start. We don't know what she was thinking when she left."

His sister had her contact lenses with her but did not take eye solution or glasses, he said.

The family was holding onto hope that she is still alive.

"I think she's probably hurt somewhere," Susan Cleverley said earlier this week.

Cleverley is white with strawberry blond hair, blue eyes and fair skin. She's 5 feet 5 inches tall and 110 pounds. Police are asking anyone with any information to call (801) 852-7241 or (801) 852-6210.

FOX News' Catherine Donaldson-Evans and The Associated Press contributed to this report.