A 17-year-old American exchange student missing in Brazil was seen trying to hitchhike to the capital the day she disappeared, Brazilian authorities said.

MyKensie Martin, a senior at Summit High School in Bend, Ore., was reported missing Sunday, said Kreg and Judy Roth of the Rotary Youth Exchange Program in Bend.

A witness spotted the girl Sunday evening on the side of a highway leading to Brasilia from the town of Unai, about 80 miles from the capital, said Unai police detective Celso Avila Prado.

Martin told the witness she was on her way to an event sponsored by the Church of Jesus Christ of Latter-day Saints in Brasilia and did not have enough money for a bus ticket, Prado said.

The witness told police the teen appeared nervous, but believed it was because she did not have enough money for a ticket, he said.

The Roths said Martin left her host city of Carmo do Paranaiba early Sunday to travel 38 miles by bus to Patos de Minas, where she attends a Mormon church.

Investigators said that instead of returning, she changed her bus ticket to travel 150 miles to Unai, Kreg Roth said Tuesday. A detective interviewed a bus fee collector who identified Martin as riding the bus all the way to Unai.

It was unclear why the teen went to Unai, though the town lies between her host city and Brasilia.

The FBI is working with its office in Brazil, as well as family members and the sheriff's office in Oregon, said FBI spokeswoman Beth Anne Steele in Portland.

"We're trying to determine exactly what happened, how it happened and when it happened," Steele said.

Martin's parents were scheduled to fly to Brazil on Wednesday. The teen left Oregon in July for a yearlong stay in Brazil.

"We're preparing for the worst and hoping for the best," the teen's father, Steve Martin, told The Bulletin newspaper of Bend. "We just feel we need to get down there because we can't effectively communicate from here and we need to be there to find out anything we can."

Unai police visited hotels in the city of 65,000 people, but found nobody else who had seen Martin, Prado said. He said there was no immediate sign that a crime was committed.

Officials with the Mormon church in Brazil referred requests for comment to a media relations office in Sao Paulo, but no one answered the phone there.

U.S. officials in Brazil are in contact with the girl's family, said Maryann McKay, a spokeswoman for the American consulate in Rio de Janeiro.

Martin's father told The Bulletin that his daughter last used her ATM card on Saturday, when she withdrew $12.

He said she understands Portuguese well enough to get around but isn't fluent.

"It's inconsistent with her character to just up and walk away," Deschutes County Sheriff Les Stiles told KTVZ-TV in Bend. "This isn't a person that does that, based on her background and how she's been in the past."