Despite recent claims by some in the bioterrorism community that the investigation should be homing in on one particular American bioweapons expert (who denies any involvement), the FBI appears to be moving in the opposite direction. U.S. government officials say the investigation is still ranging far and wide and that the FBI has not ruled out a foreign connection. The charred remains thought to belong to hijackers from United Flight 93, which crashed in a field in Pennsylvania, and American Airlines Flight 77, which smashed into the Pentagon, were examined for anthrax residue. None was found. All told, more than 5,000 interviews have been conducted and 1,700 subpoenas issued in what FBI officials say is one of the largest operations in the bureau's history. Sources tell TIME that 50 U.S. bioweapons experts have been targeted for the most intense scrutiny (including surveillance, searches of their homes and offices and even, in some cases, polygraph tests). But the pool of suspects also contains hundreds more, including researchers of biopesticides, biopharmaceuticals and veterinary products. "Remember, it doesn't have to be a top scientist. It could just be a good bench technician," says a federal investigator. Beyond the anthrax labs, the feds have also looked into more than 1,000 companies that sell equipment that could be used to process the deadly spores or that could have profited in some way from the attacks. The FBI counsels patience, but that's a tough sell to the public and increasingly vocal critics.