University of Washington police chief John Vinson said at a Thursday afternoon news conference that the man was driving a stolen truck out of Montana and had with him a scoped rifle, shotgun and fewer than 10 Molotov cocktails.
The suspect is 21, but Vinson declined to identify him.
"We have no idea what his intentions are," he said. "This is a very active investigation. We're trying to rule out everything at this point."
University police first saw a man sleeping in a pickup truck near campus late Tuesday, Vinson said. When they checked the license plate on the truck they didn't detect anything amiss.
But on Wednesday, they received a report that the truck had been reported stolen out of Montana. Officers were told to watch for the truck and spotted it on Wednesday night, again near campus.
The man was booked into the King County Jail, after campus police recognized the truck stolen from Montana.
The university police contacted Seattle police and the Federal Bureau of Investigation after searching the vehicle and finding the suspected bombs.
Vinson said the man was not cooperating with police, but said there did not appear to be any threat to the university campus.