The agency's Seattle field office, along with the Washington Joint Analytical Center, was still seeking the men's identities and whereabouts Wednesday as ferry service was temporarily shutdown when a suspicious package was found in a ferry bathroom and taken away by authorities.
"We had various independent reports from passengers and ferry employees that these two guys were engaging in what they described as unusual activities on the ferries," Special Agent Robbie Burroughs, a spokeswoman for the FBI in Washington state, told FOXNews.com.
"They felt that these guys were showing an undue interest in the boat itself, in the layout, the workers and the terminal, and it caused them enough concern that they contacted law enforcement about it," she told FOXNews.com.
The two were photographed by a ferry employee about a month ago, and those photographs were distributed to ferry employees three weeks ago by local law enforcement.
"We decided that we would go to the public to see if we could get some help in identifying them, so that we could resolve the situation more quickly," Burroughs said. "Keeping in mind that their behavior might have been completely innocuous."
Ferry service was halted Wednesday on one Seattle ferry line during the morning commute when a suspicious package was found aboard a ferry docked in the city.
The Washington State Patrol didn't disclose any details of the package that was found by a crew member in the passenger area of the Washington state ferry Puyallup, but Sgt. Craig Johnson said investigators determined it didn't pose any immediate threat and carried it off the vessel for further examination.
Johnson said no arrests were made and no identified individuals are being sought, but said the State Patrol would like to find the person who left it aboard the 460-foot Puyallup.
Marta Coursey, a ferry system spokeswoman, said the Puyallup had just been emptied of cars and passengers after arriving at Colman Dock from Bainbridge Island about 8 a.m. when the package was discovered behind the toilet in the disabled passenger stall in the men's restroom.
Two Seattle-Bainbridge runs were canceled during the package scare. Following a search and examination by State Patrol troopers, the 2,500-passenger, 202-vehicle capacity ferry was cleared to resume service about 9 a.m. During the shutdown, service on the Seattle-Bainbridge and Seattle-Bremerton runs was maintained on the terminal's other main slip.
"Just as any large transportation system would be considered a target by law enforcement, this is our big transportation system and that's why everybody here recognizes the fact that it could be a target," Burroughs told FOXNews.com. "It's something that we're very tuned into, and we monitor instances on the ferries very closely."
The FBI has received dozens of tips since releasing the photographs of the unidentified men and is following up on new leads, but they have yet to be identified as of Wednesday, Burroughs said.
The release of the photos did spark a minor controversy between Seattle's rival papers, when the Post-Intelligencer decided not to run the photo when the Times did.
"Running a photograph of two men who may as easily be tourists from Texas as terrorists from the Mideast with a story that makes them out to be persons of interest in a terrorism investigation seems problematic, to say the least," Post-Intelligencer Managing Editor David McCumber said in a blog Tuesday, noting that "ferry security is hugely important. So are civil liberties and privacy."
A Times managing editor defended the paper's decision to run the photos on Tuesday.
"Further reporting helped us more effectively weigh the potential value — and harm — of publishing the photos," said Suki Dardarian, the paper's managing editor of news coverage and enterprise. "After some deliberation, we decided to publish the photographs, along with as much context as we could bring to the story."
Anyone with information on the identities of the two men is asked to contact the FBI's Seattle office at 206-622-0460.
FOXNews.com's Sara Bonisteel and The Associated Press contributed to this report.