The report was unnerving: A woman was diagnosed with deadly bubonic plague, the first Los Angeles-area case in 20 years.
Could it be the bioterrorism attack long feared by U.S. authorities?
In Los Angeles, the FBI sprang into action, consulting with local health officials and checking the woman's name against security databases.
It soon was determined the sick woman - an El Salvadoran immigrant - was neither a terrorist nor the victim of a terrorist attack. She likely contracted the disease after being bitten by mosquitoes or fleas.
For a team of FBI anti-terrorism agents, the April incident was yet another false alarm. But in a business where U.S. authorities must be right every time and terrorists need only be successful once, everything is taken seriously.
"We operate in Los Angeles under the assumption that we will be attacked and we need to stop it," said Eric Velez, an FBI assistant special agent in charge who oversees several Southern California-based counterterrorism squads.
Indeed, since the 2001 terror attacks federal officials repeatedly have said Los Angeles is a primary al-Qaida target. Earlier this year, President Bush disclosed new details about an alleged 2002 terror plot targeting the nation's second-largest city. He said hijackers were to use shoe bombs to blow open the cockpit door of a commercial jetliner, take control of the plane and crash it into the 73-story US Bank Tower.
Velez and his secretive units are responsible for uncovering any terror plots in the region. An Associated Press reporter was given behind-the-scenes access to see how they approach their work.
Among Velez's teams is the threat squad, known as "Counterterrorism 6," or CT6, which just moved from the FBI's Long Beach office into a regional command center in Norwalk, just southeast of Los Angeles.
Every week its 22 members conduct field interviews and crosscheck databases after receiving threat reports. The unit decides if leads should be farmed out to a "substantive" squad - the al-Qaida team, for example - for long-term probes or whether they need to be dealt with immediately.
CT6 reflects the FBI's dramatic transformation since the Sept. 11 attacks. Director Robert Mueller made counterterrorism the priority, doubling the number of anti-terror agents to roughly 5,000 and forming new squads that focus more on stopping crimes than solving them.
Charged with protecting seven counties and 18 million people, CT6 was created in May 2004 after a series of reported threats to malls diverted too much manpower from other counterterrorism probes. The solution: spin off a unit to carry out initial inquiries into tips on terrorist activity.
FBI offices New York and Houston also have their own threat squads, said Gerry Brown, CT6's supervisor until last year.
CT6 follows up on about 100 tips a month that stream in from law enforcement, government officials and the public. All but a fraction have no links to terrorism.
"Some (tips) could be like, 'bin Laden is at a McDonald's drinking a Slurpee,' but that doesn't mean we don't take it seriously," Brown said.
Last month, for example, the squad got a report that three men in a vehicle had repeatedly videotaped parts of San Francisco's Bay Bridge. The concern: Terrorists might be conducting surveillance for an attack.
Following several intense hours of security database checks and phone calls, agents traced the car's owner to Riverside County. A follow-up interview at the man's home led them to rule out any connection to terrorism. The three were just tourists shooting video of a landmark.
It was typical of the type of tips the unit tracks. Other recent examples:
_ A woman reported her husband overstayed his visa and may be involved in terrorist activity.
_ Callers reported a group of "suspicious" Middle Eastern men living nearby who traveled together and sat in a car in a parking lot.