Mossad informed the FBI and CIA in August 2001 that, based on its intelligence, as many as 200 terrorists were slipping into the United States and planning "a major assault on the United States." The Israeli intelligence agency cautioned the FBI that it had picked up indications of a "large-scale target" in the United States and that Americans would be "very vulnerable." However, "It is not known whether U.S. authorities thought the warning to be credible, or whether it contained enough details to allow counter-terrorism teams to come up with a response." A month later, terrorists struck at the World Trade Center and the Pentagon in the largest terrorist attack in history.