CHICAGO_Electronic identification cards, surveillance cameras and metal detectors are fixtures in high-rise office buildings, but experts say with thousands of people whizzing through the revolving doors each day, it is impossible to guarantee workers' safety.

More than five years after the Sept. 11 terrorist attacks, security for high-rises is still a tricky balance of protecting workers and doing it without choking off commerce.

Even with all the security gizmos and gadgets, a lone gunman with a grudge against an attorney marched into Chicago's Citigroup Center last week and forced a security guard to take him to the 38th floor, bypassing turnstiles that require photo-ID cards. When he reached his destination, he killed three men and shot a woman in the foot before he was shot by police snipers.

"What can you do when someone is holding a gun on someone?" said David Hooks, spokesman for the Citigroup Center.

Chicago Police Superintendent Phil Cline says security should be up to high-rise tenants and that it is not practical to put a metal detector in the lobby of every building.

Security is not a one-size-fits-all proposition, and must include specific security threats, state and city requirements, and cost, said Santo Scribani, regional vice president for American Commercial Security Services, a Houston-based subsidiary of ABM Industries Inc.

"It's a tremendous balancing act, it's the same balancing act that law enforcement encounters," Scribani said.