The officer, working with the NYPD's firearms unit, was led into the three-story Knickerbocker Ave. building in Bushwick by a 22-year-old known gun dealer shortly before 8 p.m., police said.

As the two men stepped into the entrance, a 31-year-old man - pretending not to know the dealer - came down the stairs and aimed a loaded pistol at the officer in an attempt to rob him, cops said.

The undercover officer - who was carrying $4,200 in cash to buy a handful of weapons during the sting - pulled his gun and squeezed off three rounds at the crook, striking him in the side, police said.

"It was a preset robbery," said Deputy Inspector Kim Royster, an NYPD spokeswoman. "The firearm fell to the ground and was picked up by the unarmed guy."

The wounded bandit took to the stairs and collapsed on the second floor, where he died.

"We don't believe they fired," Royster said. "But [the officer] could have lost his life in such tight quarters."

The arms dealer ran to the third floor with the loaded, .380-caliber semiautomatic and tried to stash it near the roof, police said.

The elite backup team waiting outside saw the attempt to hide the pistol as it stormed into the building, cops said.

The man was arrested and taken into custody.

"I heard three shots," said neighbor Jackie Chow. "Three undercovers ran into the building - then, all the cops came. It's shocking. I never saw anything like that before."

Weapon of suspect who was shot and killed by undercover cop (DCPI).

Read more: http://www.nydailynews.com/news/ny_crime/2011/05/11/2011-05-11_undercover_cop_shoots_dead_man_trying_to_rob_him_in_brooklyn_source.html#ixzz1M8E4SDCN