WASHINGTON (CNN) -- The FBI earned its reputation hunting down bank robbers and other outlaws in the heydays of Bonnie and Clyde and Al Capone.

But now, as it hits the century mark, the agency's mission has changed. Instead of toting Tommy guns, robbing banks and running numbers rackets, today's Public Enemy No. 1 is more likely to be carrying box cutters, mixing dirty bomb ingredients and plotting mass casualties.

Since September 11, 2001, the FBI's priority has been to prevent another terrorist attack on U.S. soil.

CNN was given access to the Newark, New Jersey, FBI field office and top counterterrorism officials who oversee threat investigations. The routine has shifted away from the traditional cops and robbers methods.

Today's FBI focuses on vigilance and anticipating the worst.

A suspicious car drives erratically in front of an FBI field office. A different car tests positive for radiation. Military pilots flying in New Jersey say someone focused a laser on their plane. It's just another day for the Newark threat squad. Watch tracking terror all the time »

Across the nation, there are as many as 100 threats a day. Each one is checked out, no matter how small. While some inquiries are resolved quickly with a single interview, others can take months.

For example, more than 20 field offices recently investigated a man who allegedly wanted to pay cash for crop duster training, which raised red flags at the FBI because detainees in U.S. custody have talked about using crop dusters for a chemical or biological attack. After a lengthy investigation, agents determined it was a hoax -- a letter written by someone trying to damage the supposed trainee's reputation..

FBI agents are accomplished multitaskers, working on several investigations at a time. Scott Robinson, assigned to the Newark squad for four years, has been looking into the airplane laser report while also investigating a suspicious man who bought a one-way overseas airplane ticket.

"It depends on any given day," Robinson said, talking about his work routine. "But if something emergent happens, then sometimes you have to drop what you are doing." Watch investigating a threat »

Besides his own research and interviews, Robinson often seeks information from other agencies. On the days CNN followed him, Robinson talked to local police, the FAA, the Air Force, a community college, the Transportation Security Administration, a military contractor and a colleague with the Port Authority to arrange a helicopter tour of the laser incident site.

Threat information comes to the FBI in a variety of ways: from the public, other federal agencies, state and local law enforcement, overseas partners and the bureau's own intelligence sources. See photos from the FBI's history »

"The first thing we do is vet out any possible threat to public safety or anything that could possibly [put]...anybody in imminent threat or imminent danger," said Gary Adler, supervisory special agent and head of the Newark squad. Agents and analysts, part of the office's joint terrorism task force, work side by side doing everything from surveillance to background checks.

"In the big scheme of things, we have to run down every single lead to make sure that it is resolved," said Roger Morrison, section chief of the National Threat Center. "All the tips that come in are reviewed by FBI personnel. Only a handful of those every day actually have some nexus to terrorism so that we can actually drill down on those further." Watch grabbing hold of threat "right away" »

On the day CNN visited the national center, just outside Washington, officials were not only dealing with the Newark incidents but also a bomb threat in Cincinnati, Ohio, white powder reports in the Pacific Northwest and threats connected to the presidential election.

The FBI operates a "CT (Counterterrorism) Watch" in which agents and analysts operate around the clock monitoring suspicious incidents and other threats.

"If we get a name, what the intelligence analysts are doing is they're looking for other connections," said Paul Akman, assistant section chief of the counterterrorism analysis section.