An 8-year-old boy climbing on a window in the front yard of his Brooklyn home was killed on Thursday afternoon when a concrete planter attached to the house fell on him, the authorities said.

The boy, Kevin Reilly, was scaling the metal security bars on a ground-floor window about 5:25 p.m. when he apparently reached for one of the two heavy, rectangular flower boxes that were directly above the window. The planter, which rested on small bricks jutting out between the ground and first floors, tumbled on top of him, the police said.

Officers responded to a 911 call at the house in the Marine Park neighborhood and found the boy unresponsive and with head trauma, the police said. He was taken to Coney Island Hospital, where he was pronounced dead.

For hours afterward, a group of neighbors gathered near the boy’s home and expressed disbelief about the accident. They wondered whether the boy’s relatives, who they say moved into the two-story home last year, were aware of the potential danger of concrete flower boxes, a feature of many homes on the 2200 block of Ryder Street.