SEATTLE -- The Coast Guard has suspended the search for a woman who may have fallen overboard during a ferry ride from Bainbridge Island to Seattle on Monday.

Washington State Ferries spokesperson Susan Harris said an empty car was found aboard the ferry Tacoma after it docked in Seattle around 2:55 p.m.

The discovery prompted the search for the driver of a gold 1999 Mitsubishi Galant, whom they believe is a woman. The car is registered to Amy Story, 27, of Sequim, Wash.

"The vessel crew followed their normal protocol," said Sgt. Craig Johnson. "Did an additional sweep of the vessel, checking all the compartments from top to bottom. Weren't able to locate anybody aboard the vessel."

Using both boats and a helicopter, the U.S. Coast Guard searched the ferry route for several hours Monday night, but found no sign of the missing woman. It suspended the search just before 11 p.m.

It is not unheard of for a passenger to walk off a ferry, forgetting they'd driven their car that day. Ferry officials said a passenger makes this mistake once every two or three months.

But state troopers said they found the woman's purse, wallet and cell phone inside her car.

Amy's sister told KOMO News Story suffers from mental illness and could seem dazed and may be talking to herself. She's hoping people in Seattle might recognize her photo if she just wandered off the ferry.

It is not known whether any other ferry passengers noted the missing woman's movements.

The temperature of Puget Sound is approximately about 50 degrees. In such temperatures, the average person would likely lose consciousness or become exhausted after about a half-hour to an hour, according to experts.