The 38-year-old man walked into the classroom just after 1 p.m. and soon began making a scene.
"He told her, 'Keep teaching. Keep teaching. I want to learn,' " said student Yen Jones, who captured part of the incident on her cellphone camera.
Professor Madeline Kass also realized that something was wrong with the man.
"He's acting very strangely by dancing, jumping on the table, waving an ice cream cone around," she said.
The man was asked to leave, but ignored the request. Cell service in the basement classroom is spotty, so a student stood up to try for a better reception. That's when the intruder began to really frighten the students.
"The man yells 'Bam!'" Jones said. "And so she obviously she thought it was a gun, I think, so she squatted back down. I thought it was a gun at first."
Kass used the classroom's emergency button to call for help, but there are differing opinions about how long it took for campus security to arrive.
"We responded within one minute of getting the call in the dispatch center," said Randy Carroll, the school's interim public safety director.
Some students say it took longer than that.
"We definitely relied on our school to make us feel safe, and I know I didn't feel safe and I can tell you for most of my peers felt terrified in that moment," Jones said.
Kass said it took 2 to 5 minutes for security to arrive, but said the tense situation made it feel like longer.
"The context of everything going on lately in the news and things happening at schools was enough to make anyone wonder what was going to happen it was scary," she said.
The intruder, who has not been identified, slipped away from security guards but was arrested by Seattle police a short time later at nearby chapel.
He was booked into King County Jail on suspicion of trespassing. Police believe the man may be suffering from mental illness.