BALTIMORE, Maryland (CNN) -- The informant who told U.S. authorities about the purported plan to set off explosives in a Baltimore Harbor tunnel is a prisoner in the Netherlands, two federal officials confirmed to CNN on Wednesday.The prisoner underwent a polygraph test last weekend, and some of the answers were considered deceptive, the officials said, without offering further detail.One federal law enforcement source said the test was "inconclusive." The informant has provided some useful information to authorities in the past, a U.S. government source has told CNN.Authorities said the investigation into the terror tip continues, but that no plot has been uncovered. Agents continue to look for suspects, sources said.Four men who sources previously had said were detained in connection with the investigation will be deported on immigration charges, an Immigration and Customs Enforcement official said Wednesday.The four are Ahmad Al Momani, 58, a citizen of Jordan, and three Egyptian citizens -- Mohamed Ahmed Mohamady Ismail, 30; Mohamed Mohamed-Abdelhamed Elsyid, 38; and Suied Muhamad-Ahamad, 25, said Ernestine Fobbs, a spokeswoman for ICE.No information was provided on their connection to the Baltimore case, if any. The plot, as outlined by the informant, involved detonating at least one truckload of explosives in a Baltimore Harbor tunnel, government sources told CNN. The explosives were to arrive aboard ship containers labeled cocoa. As many as eight Egyptians were involved, the government sources said. The attack was to take place sometime during a two-week period this month, federal and state officials said.The informant phoned the tip to U.S. authorities late last week, sources familiar with the investigation told CNN on Tuesday. Law enforcement and counterterrorism agents and analysts immediately started trying to corroborate the plot and locate the alleged perpetrators.While there were questions about its credibility from the start, officials said they still had to take the information seriously and started operations midday Tuesday to locate and interview people who could be involved. At the same time, authorities closed down one tunnel under Baltimore Harbor and limited traffic in another.CNN's Pam Benson, Kevin Bohn, Carol Cratty, David Ensor, Terry Frieden and Jeanne Meserve contributed to this report.