PORTLAND, Ore. -- Investigators are trying to determine what caused a fire that has forced hundreds of elementary students to move to another school.

A three-alarm fire caused significant damage to Marysville Elementary School mid-day Tuesday.

The cause of the fire remains a complete unknown, officials said, but the school is being considered a crime scene until investigators can rule out arson. Federal investigators from Alcohol, Tobacco and Firearms were at the scene Wednesday morning.

The fire started just before noon Tuesday and the school was quickly evacuated. All 460 students and 17 teachers were quickly accounted for, said Kim Kosmas in a Portland Fire and Rescue news release.

Two adults suffered minor smoke inhalation, fire crews said, and no children were injured.

Witnesses said there were 100 firefighters on the scene. The fire spread to the attic and roof and it significantly damaged at least one-third of the school. The thick, black smoke was visible from downtown Portland.

Some students initially believed it was another fire drill.

"I thought it was just a fake until we got outside and smoke and flames were coming out," said Mackenzie Nizzi, a second-grade student.

"I'm sad that the school burned down because they just built a new playground," said Anthony Hewgley, a fifth-grader. "(But) I'm happy cause we don't have to school."

Marysville Elementary School, located at 7733 SE Raymond St., houses students in kindergarten through 8th grade. The school building was built in 1922.

Amber Stenseth, the school's head custodian, said the fire mostly burned kindergarten and 3rd grade classrooms. She said the fire alarm went off during lunch period and the children filed out in an orderly fashion.

"I went to the office to check to find out where it was coming from and where it was going off and our principal came running down saying, 'It's burning from the Discovery Zone.' And that, 'This is not a joke we need to get out,'" Stenseth said.

Stenseth also said the fire may have started in the Discovery Zone, a hands-on classroom that allows students to learn about physics and engineering.

Robb Cowie, a Portland Public Schools spokesman, said he did not have any information about how the fire started.

Parents were told to pick up their children at the Holgate Public Library, which was accessible from Southeast 82nd Avenue. Staff members stayed with students until they were picked up by parents.

Beginning Monday, district officials said Marysville Elementary School students will go to Rose City Park School, located at 2334 NE 57th Ave. Rose City Park School closed two years ago.

Portland schools will be closed Wednesday for Veterans Day. Marysville students won't go to school on Thursday or Friday and teachers will have planning days on those days.

Slideshow: 3-Alarm Fire Burns Portland School