At an AvalonBay Communities leasing office in early June, a package arrived for a resident who wasn't home. It was ticking.

Office staff called the local police, who showed up with a bomb squad, X-rayed the package, and opened it carefully. Inside was a clock, which the absent tenant bought from a mail-order catalog. The well-intentioned sender had placed a fresh battery in it before shipping it.

"It was a good ending," says Laura Novak, senior director of risk management for AvalonBay Communities, the Alexandria, Va.-based owner of 40,000 apartment homes–mostly garden-style–in 140 locations. But if the suspicious ticking had turned out to belong to a bomb, it would have been in appropriate hands–those of the local police, she says.

Indeed, employees of AvalonBay Communities are quick to involve local authorities if they receive suspicious packages, observe suspicious behavior, or find suspicious items around their communities or even inside tenants' apartments. And they know exactly what "suspicious" means, thanks to training–mostly via memos from Novak's office–that has made them hyper-aware of the possibility that terrorists will either target an apartment building or move into one and plan their attack as their neighbors fix dinner or drift off to sleep.

The FBI has warned that apartment buildings, especially urban high-rises and landmarks, could be terrorist targets. The U.S. Department of Homeland Security has asked landlords to notify it when tenants display behaviors associated with terrorism.

The U.S. Department of Justice drove both points home at around the same time AvalonBay had its ticking-package scare, when it released hair-raising details about an alleged al Qaeda plot involving suspect Jose Padilla to blow up multiple high-rises. Padilla and an accomplice, according to U.S. Deputy Attorney General James Comey, planned to rent apartments in buildings with natural gas heat, "seal those apartments, turn on the gas and set timers to detonate and destroy the buildings simultaneously at a later time." The original plan was to blow up 20 buildings at once, possibly in Washington, D.C., New York, Chicago, Florida, Texas, or California.

In response, managers and owners of apartment buildings and communities have enlisted maintenance crews, office staff, the local police, and even tenants as the "eyes and ears," says Roger Platt, senior vice president of the Washington, D.C.-based Real Estate Roundtable, "but they have to do it in a way that complies with laws."