One started off as an auxiliary officer, the other as a patrolman. Neither knew they would one day end up as the first line of defense against terrorist threats in New York City.

“I did not envision this when I joined the force,” Inspector Gary Scirica, 48, the counterterrorism coordinator for the Queens South patrol borough, said. “I thought narcotics, drug dealers, crooks. I couldn’t have imagined this assignment.”

A few minutes earlier, he told six captains huddled in the back of a mobile police communications van near Battery Park: “We need to look everyone in the eye. Tell your officers to engage the surroundings. This is not a drill.”

Outside, about 170 police officers from every one of the city’s 76 precincts sat in their patrol cars with lights flashing, awaiting orders. The deployment, called the Critical Response Vehicle “surge,” is one of the more concrete daily actions of the New York Police Department’s counterterrorism and intelligence divisions. Using intelligence culled from a range of agencies — from the NYPD’s own sources to CIA and FBI bulletins — the officers swarm around areas and structures of the city that may be vulnerable to a terrorist attack. They ask questions and look out for suspicious activity. Last Wednesday’s targets included the Fifth Avenue Synagogue, Times Square, and Midtown subway stations.

Around the same time Inspector Scirica was sending out the critical response vehicles in a blaring, showy display of force, Lieutenant Christopher Higgins, 44, was sending his detectives to businesses around the city from his office at the Brooklyn Army Terminal.

The intelligence detectives visit ambulance companies, storage facilities, scuba rental stores, photo processing shops, and hospitals, among other locations, as part of Operation Nexus. Each visit takes about 15 minutes, and detectives let the owners know what kind of suspicious activity to look out for and what number to call if they see something. About 30,000 businesses have been visited since September 11, 2001.

“We act out of an abundance of caution,” he said. A beauty salon recently called the terrorism hotline after a man bought a gallon of hydrogen peroxide, which can be fashioned into a bomb when combined with products available at grocery stores. It turned out that he wasn’t planning any criminal activity, but the call itself was a small success, Lieutenant Higgins said. It meant that businesses were watching out for the city.

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Raymond Kelly became police commissioner for the second time two months after the World Trade Center attacks, and he set to work immediately on refashioning the department to cope with a new kind of threat. Today, 1,000 officers and hundreds of civilians are devoted to the terror beat. They are led by Deputy Commissioner of Intelligence David Cohen, who spent 30 years in the CIA, including a two-year stint as the director of operations, and Deputy Commissioner of Counterterrorism Richard Falkenrath, who is the author of books about the threat of chemical and nuclear weapons and was one of President Bush’s counterterrorism strategists. The department is budgeted to spend about $135 million on counterterrorism this year.

At a recent briefing, Mr. Kelly told the city’s private security directors: “Quite frankly, we see no letup in Al Qaeda’s activities. We’re going to be in this fight for many, many decades to come.”

Mr. Falkenrath called terrorism a “permanent condition, at least for my lifetime.”

***

Inspector Scirica joined the police department as an auxiliary officer in Queens in 1980, and Lieutenant Higgins as a patrolman in the Queens in 1985 — a period when the murder rate reached an all-time high, and drugs proliferated on the streets.

Inspector Scirica, who has been a commanding officer of Queens Narcotics, as well as the 100th and 101st precincts, said that while terrorism is a new breed for local law enforcement, it is still, at its heart, just a crime.

“The same way I might shake down some sources in a narcotics investigation, I might do in a terrorism investigation,” he said. “A lot of the same tactics apply.”

Unlike narcotics crime, which is tracked daily by the department’s computerized statistics program, Comp-Stat, there is no way to measure how well the city is protecting itself against terrorism except by the fact that nothing is happening. It makes for tense crime fighting because the perpetrators may be invisible or they simply may not exist, Inspector Scirica and Lieutenant Higgins said.

When Lieutenant Higgins first started in the intelligence division in 1999, the scope was radically different.

“Before 9/11, terrorism was about domestic threats, mostly radical environmental groups,” he said. “And even then, there wasn’t a lot going on in New York City.”

He was in charge of putting together a daily briefing for the commissioner, but the information mostly came from open sources like newspapers. Now, the city’s eight counterterrorism coordinators, one for each patrol borough, receive real-time updates on conditions around the world and the city. The coordinators then create bulletins for precinct leaders, who brief patrol officers at roll call. The police commissioner’s morning briefings now include classified information from the federal government, as well as information from nine detectives stationed overseas in places such as Tel Aviv, Paris, and London.

The route for the CRV deployment that Inspector Scirica coordinated was designed the night before by the intelligence division, but Lieutenant Higgins said that if some intelligence suggested a structure in the city was at risk or that terrorists might be conducting surveillance of a certain area, they could redeploy quickly.

“We respond on a dime now,” he said. “Before September 11th, it would take hours to harness those resources and deploy them.”

With a terror threat so amorphous and unquantifiable, the department spends most of its counterterrorism efforts on securing infrastructure and other preventive measures, and those efforts are not restricted to New York City.

Lieutenant Higgins sends detectives to businesses that sell potentially dangerous material in Pennsylvania, Connecticut, New Jersey, and upstate New York. He personally has visited about 20 trade associations in Washington, D.C., who agreed to either send out an e-mail detailing the police department’s concerns or printing an advertisement in their newsletter with terrorism indicators and the phone number for the NYPD hotline. Earlier this year he attended a convention of regional crop dusters in Kansas along with the Transportation Security Administration and the FBI.

“The next Mohammed Atta is likely to interact with a business rather than a law enforcement type,” he said. “These groups are trained to hide in plain sight.”

In addition to the CRV surge, the police department deploys Hercules teams — made up of Emergency Service Unit officers in full gear, canine units, as well as harbor and aviation units — to guard parts of the city. In the subways, police conduct random searches of bags, occasional explosives tests, and Total Order Maintenance Sweeps, during which a train is held while a dozen officers investigate check for suspicious packages and activity.

“If you told us in 1985 that we would be fighting terrorism of this magnitude, we would be incredulous. We couldn’t comprehend,” Lieutenant Higgins said. “If you told me that when I look out of One Police Plaza, to the left of the Municipal Building, there would be a void. …”

“The changes, they’ve been seismic,” he said.