There have been a number of suspicious incidents this summer aboard Washington state ferries, which prompted the Department of Homeland Security (DHS) and the Coast Guard to increase security along the ferry lines and to issue a warning to law enforcement. "On several occasions since May 2007, members of the public and employees of the Washington State Ferry (WSF) have reported a number of suspicious activities aboard state ferries," said the note, sent out Wednesday to state and local law enforcement by the chief intelligence officer at DHS. The warning was issued as the hunt continues for two potential suspects that were observed on multiple Washington state ferries. The individuals "exhibited unusual behavior and undue interest in the layout and workings of the ferries and ferry terminals," said the note. THE BLOTTER RECOMMENDS Blotter Exclusive: U.S. Studying Two Dozen ‘Clusters’ of Possible Homegrown Terrorists Blotter FBI Urges Vigilance With 9/11 Anniversary in Sight Blotter American Al Qaeda Leader Says U.S. Embassies a Prime Target Click Here for Full Blotter Coverage. Security is always high on the ferries, according to a spokesman for the Washington State Patrol, but current security levels are no higher now than usual. The spokesman said all of the incidents still considered suspicious involved the same two individuals who authorities are now searching for. The FBI released photos of the two men on Monday. Click Here for Full Blotter Coverage. Ferry employees, however, also often question passengers who seem more interested in the inner-workings of the ferry than the scenery, but most of those incidents are resolved on site. Just Wednesday, service on one ferry was suspended for an hour when a ferry worker found a suspicious package during a routine sweep. A bomb squad was called to the scene and determined the package was not a threat. Al Qaeda has demonstrated an interest in various types of maritime targets. They were, of course, successful in October 2000 with the attack on the USS Cole in the Gulf of Aden. "Many ferries operate in close proximity to energy-related critical infrastructures, such as petroleum and chemical facilities and power plants," a law enforcement source told the Blotter on ABCNews.com, "making them attractive targets to terrorists." While DHS has increased security on the ferries and ferry facilities, there is no credible intelligence indicating a specific threat to the ferry system in Washington state or anywhere else in the U.S., a spokesman for DHS told ABCNews.com. Do you have a tip for Brian Ross and the Investigative Team?