The 21-year-old Nevada man, whose name was not released, was arrested near the University of Washington campus Wednesday, The Seattle Times reported.
University Police Chief John Vinson said officers first contacted the man Tuesday when they found him sleeping in the truck, but they released him after finding no warrants.
Police were notified Wednesday the truck was stolen in Montana, and they arrested the driver about 10:30 p.m. A search of the truck allegedly turned up a stolen scoped rifle, a stolen shotgun and about 10 Molotov cocktails.
"We have no idea what his intentions are," Vinson said of the suspect, who was booked into King County Jail.
"At this point, there does not appear to be any threat to the University of Washington community. This is an ongoing criminal investigation," Sgt. Sean Whitcom said.
The FBI joined the criminal investigation.