PRINCETON, N.J. (AP) _ Police interrupted a bank robbery and fatally shot a ski-masked man who was holding a gun to an employee's head. Two other robbers escaped with another hostage who was later found unharmed.

Police were called by a repairman who had been working on an automatic teller machine at a Sovereign Bank across from Princeton University on Thursday evening. He told a 911 operator that he saw the employee behind the counter with her hands bound.

Officers arrived and encountered a ski-masked, rubber glove-wearing man walking out of an elevator, holding a woman by the neck and pressing a gun to her head.

When officers told him to drop the gun, he screamed at them and waved the gun, threatening to kill the woman. As he brought the gun back to her head, the officers opened fire, killing him. The employee was not injured.

Two other robbers with another hostage jumped into a car behind the bank and drove away, Police Chief Thomas Michaud said. But they crashed the car, abandoned it and left the hostage.

They then hijacked a second car after pushing its elderly driver out but later abandoned that car, too, Michaud said. Police said they found a large sum of money inside but declined to say how much.

Police scoured the normally quiet, prosperous neighborhood after using a helicopter to search for suspects. The university locked its dormitories early and warned students by e-mail and posters around the campus, spokesman Justin Harmon said.

The hijacked car was driven by Lucius Wilmerding, 91, a historian who was a visiting scholar at the prestigious Institute for Advanced Study in Princeton from 1944-48.

Wilmerding's son Walter said police told him his father had the presence of mind to drive the car with the emergency brake on to slow the bank robbers down, The Star-Ledger of Newark reported.

``My father is pretty unflappable at his age,'' Walter Wilmerding told The Trentonian. ``He drove away as slow as he could and that ticked off the robber. The guy threatened to kill my father.''

After being pushed out of his station wagon, Wilmerding was taken to The Medical Center of Princeton, where he was in fair condition today, said hospital spokeswoman Carol Norris. She had no immediate information on the nature of his injuries or when he might be released.