US federal security officers arrested an Israeli and four Angolans on Sunday, on suspicion of plotting an attack on a passenger plane at Newark Liberty International Airport. The five were traveling back to their home countries on Saturday after attending helicopter training school in Texas, said Steven Siegel, a spokesman for the FBI's Newark office. Fellow passengers on Flight 1874, which had departed from Dallas Fort Worth International Airport, became suspicious of the men because they were speaking in foreign languages and had "aircraft flight materials" with them, Siegel said. A federal marshal on the plane notified authorities at Newark Liberty about the men's behavior. Police officers from the Port Authority of New York and New Jersey, which operates the airport, took the men into custody soon after the plane landed around 3:20 p.m., said a Port Authority spokesman. The men had no weapons. After being questioned by authorities, the five were found to pose no threat and were released, an FBI spokesman said.