Concrete barriers on a truck arrive at Warner Bros. Studio, which will be used as a security barrier, on September 21 in Burbank. Major Hollywood studios tightened security, cancelling tours and setting up metal detectors and barriers after the FBI warned they could be the ``target of a terrorist bombing.'' FBI spokesman John McLaughlin said the agency met with the heads of major studios on September 20, hours after receiving an ''uncorroborated yet credible'' threat against the filmmakers. — REUTERS