Searchers found the black box of one hijacked airliner in Pennsylvania and received a signal from the black box of the plane that crashed at the Pentagon, officials said Thursday.

Attorney General John Ashcroft said the FBI was working on "thousands and thousands of leads" in the investigation of Tuesday's terror attacks. Search crews will not be able to retrieve the black box at the Pentagon, which could contain information about the last minutes of the hijacked commercial jetliner, until they are able to enter the collapsed area of the Pentagon, where the plane's fuselage rests. They were to begin moving into the collapsed area sometime Thursday night, said Arlington County Fire Capt. Scott McKay. While there have been no arrests, Ashcroft said, authorities have interviewed many people in connection with the hijacking of four airliners and the attacks on the World Trade Center in New York and the Pentagon. A total of 18 hijackers were on the planes, Ashcroft said. There were five on each of two planes and four each on the other two. All have been identified, officials said. He said he had was heartened by the public's interest in tracking down those responsible. About 40 of the people involved in the attacks have been accounted for, including those killed in the suicide attacks, but 10 remain at large, the Los Angeles Times reported, citing an unidentified source with knowledge of the investigation.