A male in his 20s has died and three others are injured after a shooting at the campus of Seattle Pacific University, a hospital spokeswoman said.
Police said on their verified Twitter account on Thursday that a lone suspect entered a university building, shot two males and two females, and began reloading.
The gunman was arrested after he was disarmed by university staff, police tweeted.
The university announced online that it was evacuating people and that "the campus is in lockdown due to a shooting in Otto Miller Hall".
One student recounted hearing a loud bang from next to the classroom he was in. His teacher initially thought it was a science experiment, but someone went out and came back saying "I think someone's been shot."
"So we turn off the lights we lock the door and we close the blinds," the student, identified as Blake, told KIRO 7 TV.
After a couple of minutes, "cops come in through another classroom which was connected to ours and they escorted us out.
"I just saw piles of blood on the ground, just scattered throughout the lobby."
The shooting came less than two weeks after a mentally unstable 22-year-old opened fire at a campus in Santa Barbara, California, killing six people before turning the gun on himself.
Other school shootings in the US in recent years include the December 2012 massacre in Newtown, Connecticut that left 20 small children dead, and the Virginia Tech college shooting in April 2007 in which 33 people died, including the lone gunman.