Mr. Kessler said that while every building has unique strengths and weaknesses, there are a few broad categories to be examined. The most obvious, he said, is the physical security of the building itself. Basically, Mr. Kessler said, this means preventing unauthorized individuals from gaining access to the building and to apartments within it.

Another critical element, he said, is to make sure that those who are authorized to be in the building do not themselves pose a threat to residents. That, Mr. Kessler said, can be accomplished by conducting background checks on employees and perhaps even new residents.

And a third element of a safety and security plan — emergency management and evacuation planning — has less to do with preventing problems than with responding to those that occur.

"Everyone seems to be a little more vigilant now," Mr. Kessler said. "And we're seeing a move on the residential side to security strategies that have been in place in commercial buildings for years."

Michael Cherkasky, president and chief executive of Kroll Inc., a Manhattan-based international security firm, said that for a co-op or condominium board to assess a building's needs, it must first determine the building's security profile. "Some buildings have higher security profiles than others," he said, explaining that buildings that are close to potential targets of terrorism — government offices, foreign consulates, and even, perhaps, churches and synagogues — will have higher security profiles than buildings that are not near such targets.

And people who live or work in a building can have an impact on the building's security profile. "You can have one tenant in the building whose presence changes the profile of the entire building," Mr. Cherkasky said, pointing out that government officials, diplomats, corporate executives and certain celebrities can increase the need for tighter security in a building.

"So the first thing a co-op or condominium board should do is get an assessment of the types of risks their particular building is exposed to," he said.

Dave Aggleton, president of Aggleton & Associates, a Manhattan-based security consulting company, agreed.

"It is imperative for boards and building owners to analyze security needs before developing security solutions," Mr. Aggleton said. "Most residential buildings do not have particularly high levels of security. But the events of Sept. 11 have provided a good opportunity for people to reassess what their security needs really are."

Mr. Aggleton, who is also vice president of the International Association of Professional Security Consultants, a trade organization based in Buena Vista, Calif., said that the most obvious place to start with a physical security plan is at the front door.

"You can always lock the front door," he said. "But keys are very easily duplicated."

An alternative to keyed locks on the exterior front door — or an interior vestibule door — is the magnetic card-type device used in many hotels. "The most common card reader now is the proximity reader," he said, explaining that instead of requiring a card to be slid into a slot, a proximity reader will unlock the door when the card is held close to the reader's electronic sensor. Even those devices, however, are not foolproof. "The card reader is only checking the card, not the person who is using it," Mr. Aggleton said.

In fact, he said, the most secure entry systems available today are "biometric access control systems." Such systems, Mr. Aggleton said, recognize some unique feature about the person attempting to gain access. "There are a number of different types of biometric devices," he said. "Some will do a retinal eye scan, some will scan the iris, some will recognize a fingerprint and some will do hand geometry checking."

Andsome biometric devices are not nearly as expensive as one might think, he said. Available for as little as $900, "fingerprint readers are only about three times the price of a regular card reader," he said. And while additional equipment is needed to install an entire system — including a controller panel, a computer, software and an electric locking mechanism on the door — it is possible to install a state-of-the-art biometric access control system in a building for less than $10,000.

Mr. Aggleton added that all potential points of access to a building need to be secured. For example, he said, if a building has a service entrance, it might be wise to have an electric locking device on the door that can be opened by a building employee in the lobby who is monitoring the service entrance over a closed circuit television camera. It is also possible to install cameras in the vestibule that make it possible for occupants to see who is calling on them without leaving their apartments.

Gregory J. Carlson, executive director of the Federation of New York Housing Cooperatives and Condominiums, said that one way for a building to increase security without installing expensive electronics is to have a human being stationed in the lobby. In some buildings, Mr. Carlson said, a doorman might be sufficient. In others, the doorman could be supplemented with a uniformed security guard. "You can have a guard on duty for one shift, two shifts, or on a 24-hour basis," he said. "You just tailor the contract to meet the needs of the building."

Security guards, who usually cost $10 to $15 an hour, can be useful even in buildings with a doorman. "You can use the security guard as an escort for when people come into the building to look at apartments for sale," Mr. Carlson said, adding that in large buildings or buildings with parking garages, additional guards can patrol different parts of the building or the garage. "In the building where I live, we have an extra guard who comes in at night just to watch the garage," he said.
Mr. Kessler of Kessler International said that even when access to a building is tightly controlled, it is still necessary to ensure that those who have been granted access do not themselves pose a security risk.

"A lot of boards are asking us to do background checks of new employees and people moving into the building," Mr. Kessler said, explaining that such investigations typically involve following up on information about the individual's past that has been provided by the individual. "We will go out and interview people they've worked for and speak to people in places where they've lived," he said.

In some cases, Mr. Kessler added, public records will be checked to determine if a prospective employee or tenant-shareholder has a record of any criminal convictions. "There are different levels of background checks you can do," he said, adding that such an investigation can range from a few hundred to a few thousand dollars.

A critical element of any building security plan is to have a plan in place in the event an emergency occurs. Daniel J. Satoma, a Manhattan lawyer who specializes in emergency management, said that since the safety of building residents is ultimately the responsibility of the building owner — whether that owner is an individual or a corporation — it is critical for owners and co-op and condominium boards to make sure that there is an emergency evacuation plan in place.

"Tenants should understand exactly how they should be exiting the building," Mr. Satoma said. "They should know where the emergency exits are located, they should know that some of the building systems may not be working, and they should know where to go upon exiting the building."

In addition, Mr. Satoma said, a building's management should determine whether there are any tenants in the building who will require assistance getting out and then make plans for providing such assistance.

"Communication between residents and building management is critical," he said. "Owners and boards who understand that will have an opportunity to create a safer and more secure environment for the people in their buildings."