Lauren Boushey, 20, a junior at Western Washington University in Bellingham who was at the apartment complex party, said it broke up around 9 p.m. and police officers politely asked people to go home.
"It was set up to be ... this really nice night and nothing reckless or ridiculous like it turned into," she said Sunday, noting that the party wasn't connected to the ensuing ruckus. People lingered and a crowd started forming a block away, she said.
Many of the revelers moved to nearby Laurel Park, where they were joined by even more people, Scanlon said, calling it a "large, intoxicated, disorderly crowd."
The situation "finally boiled over," he said. "They began hurling projectiles at police."
Up to 500 people had converged on the park as police worked to disperse them, at one point using pepper spray, Scanlon said.
It took about 45 minutes to clear the area and restore order.
Boushey said she saw several people throwing bottles and beer cans at police officers and provoking them. She saw multiple officers get hit by bottles.
"They got this horrible ignorant mob mentality," she said. "It was so sad and disrespectful to watch."
She said there was a large police presence during the melee and at one point, what she called a "riot tank" rolled through with police officers hanging off the sides and loudspeakers warning people to disperse.
"It was very surreal," said Victoria Sewell, a WWU senior, who walked into the fracas Saturday night on her way to a friend's house. "There were so many police cars and people. I've never seen anything like that in Bellingham."
She said police let off tear gas and sound grenades.
Scanlon said there were multiple arrests, but he didn't have an exact figure. He said he didn't know whether there were any injuries.
The university released a joint statement from WWU President Bruce Shepard and Western Associated Student Body President Carly Roberts on Sunday afternoon saying three arrests were made and none was of a Western student. They also said no serious injuries were reported.
The statement said law enforcement will review surveillance videos and criminal charges may follow. Officials at the school of about 15,000 students will also review the incident and enforce the student conduct code.
Shepard and Roberts said they were thankful that students were among those who stepped up to clean up the area.