NORWALK, Calif. » The report last month was chilling: a 55-gallon drum of radioactive material had gone missing during shipment from North Carolina to California. Even worse, the person who signed for the cargo was not an employee of the company that ordered the load.

The Federal Bureau of Investigation here ramped up, consulting health officials, quizzing radiation specialists and tracking down the trucker who dropped off the material that could be used in a radioactive-bomb attack. Three hours later, the shipper found the drum — still sitting on a loading dock 20 miles from its destination in the Los Angeles area — having confused it with a similar shipment sent to a different company on the same day.

For an FBI team here that vets tips and threats about possible terrorist activity, it was yet another false alarm in a job largely defined by hoaxes and bogus leads that must still be run to ground. “;A lot of time we are chasing shadows,”; said Lee Ann Bernardino, a 20-year FBI special agent who handled the case, “;but it's better to do that than find out later you let something get by.”;

Spending two days with Bernardino's 21-member threat squad, known as “;Counterterrorism 6,”; or CT-6, offered a rare window on the daily workings of an FBI transformed after the attacks of Sept. 11, 2001. The bureau now ranks fighting terrorism as its No. 1 priority. It has doubled the number of agents assigned to counterterrorism duties to roughly 5,000 people, and has created new squads across the country that focus more on deterring and disrupting terrorism than solving crimes.

But the manpower costs of this focus are steep, and the benefits not always clear. Of the 5,500 leads that the squad here has pursued since it was formed five years ago, only 5 percent have been found credible enough to be sent to permanent FBI squads for longer-term investigations, said Supervisory Special Agent Kristen von KleinSmid, the head of the squad. Only a handful of those cases have resulted in criminal prosecutions or other law enforcement action, and none have foiled a specific terrorist plot, authorities acknowledge.

As part of the larger debate about the transformation of the FBI, some counterterrorism specialists question the value of threat squads — which are also in Washington, New York and a few other cities.

“;Just chasing leads burns through resources,”; said Amy Zegart, associate professor of public policy at the University of California, Los Angeles, who writes extensively on intelligence matters. “;You're really going to get bang for the buck when you chase leads based on a deeper assessment of who threatens us, their capabilities, and indicators of impending attack. Right now, there's more chasing than assessing.”;

The FBI director, Robert S. Mueller III, has acknowledged the toll of the shift of agents to counterterrorism and intelligence duties. It comes at the cost of resources to combat corporate and financial fraud, and the deadly drug war in Mexico. About 40 percent of the bureau's agents are devoted to fighting terrorism.

The threat squad here is just one part of the FBI's Los Angeles field office that focuses on fighting terrorism. About 30 percent of the office's 750 agents work on terrorism cases, involving al-Qaida, Hamas, terrorist financing and animal rights extremists.

Federal agents say a major lesson of the Sept. 11 attacks is that all credible reports of possible terrorist activity must be checked. And they say it is more efficient for one squad with specially trained investigators to assess these tips, allowing other agents to stay focused on longer-term terrorist inquiries.

The squad's work here has yielded important results, officials say. In March 2008, Seyed Maghloubi, an Iranian-born American citizen, was sentenced to 41 months in prison for plotting to illegally export 100,000 Uzi submachine guns to Iran, via Dubai. His arrest stemmed from a tip from a police informant whom Maghloubi contacted about buying the weapons. The threat squad picked up the tip and developed information that led to a federal sting operation against Maghloubi.

Responsible for overseeing seven counties and 19 million people in Southern California, the threat squad was created in May 2004 after threats to shopping malls in the west side of Los Angeles diverted about 100 agents from other counterterrorism inquiries. Working out of a drab office building here 15 miles southeast of downtown Los Angeles, the investigators sift through tips and threats called in by the public or passed on by a regional intelligence center. The agents check databases and conduct field interviews before deciding whether to act on a case immediately, farm it out to another FBI squad, or refer it to another law enforcement agency.

“;Someone has to go out and knock on the doors,”; said Frank Leal, a 29-year detective with the Los Angeles County Sheriff's Department who is assigned to the threat squad along with investigators from 10 other local, state and federal agencies. “;You don't want any one of those leads to go boom.”;

The squad now gets about 80 leads a month, down from a peak of about 140 a month a few years ago, a decline von KleinSmid attributed in part to greater screening of tips by other intelligence analysts.

Recent reported threats range from the mundane to the bizarre.

On Aug. 1, a man called in a bomb threat to a Marriott-chain hotel in Hollywood. Authorities found nothing in a sweep of the hotel. A few hours later, the same man called to ask if the hotel had by any chance lowered its rates recently, and if it would do so if a bomb threat came in.

Security guards have questioned people taking pictures of oil refineries in the Los Angeles area. Many turned out to be college students fulfilling assignment for class projects.

Another recent reported threat sounded like a Hollywood thriller. In June, a college student told her University of California, Riverside, professor that her father, a Pakistani microbiologist, was secretly testing botulism toxins on animals in their basement on the outskirts of Los Angeles. FBI agents, backed by police and hazardous-material experts, moved in on the house only to find nothing. The student had been trying to impress her professor in a very weird way, investigators said.

Nicholas M. Legaspi, the lead FBI special agent on the bogus biolaboratory case, said he had no regrets about the effort devoted to the false alarm, which he said served as an excellent training exercise.

Legaspi said his initial frustration about working on the threat squad was tempered by overseas assignments in which he investigated the attacks in Mumbai, India; worked alongside American Special Forces in Afghanistan; and interrogated Qaida detainees at the American prison at Guantanamo Bay, Cuba. “;For the first several years, it was very disappointing always chasing ghosts,”; said Legaspi, who is a former California Highway patrolman and Army cavalry officer. “;But looking at what goes on overseas keeps me sharp. I realized the terrorists are deadly serious. It makes me hungry to do this job.”;