The driver, a woman from Houston, was taken into custody but released after interviewers determined she hadn't known the grenade was in the car, Royal Canadian Mounted Police spokesman Tim Shields said.

Shields said the 28-year-old woman apparently was lost, and that she meant to drive to Vancouver, Wash. - at the state's border with Oregon - not Vancouver, British Columbia, 250 miles to the north.

The woman's husband works for the U.S. military and is believed to be stationed at Fort Lewis, Wash., he said. [Tony's note: what was a soldier doing with a grenade in his off-base civilian vehicle?"]

Shields said the woman was quite shaken by the incident, and added that charges are unlikely.

Canadian customs spokeswoman Paula Shore said the grenade was discovered about 1 p.m., prompting the closure and evacuation of the Peace Arch border crossing in Blaine. It reopened at 2 p.m., and the woman was returned to the United States.

During the closure officials rerouted traffic to the truck crossing on Pacific Highway, less than a mile to the east.

Mike Milne, a Seattle-based spokesman for the U.S. Department of Homeland Security, confirmed the closure but declined to elaborate.

About 8 million vehicles cross at Blaine, the northern end of Interstate 5, every year.