With a loaded handgun tucked into the waistband of his bluejeans, the authorities said, Demetrius Blackwell walked the familiar blocks near his home in Queens Village on Saturday.

As a boy on those same residential streets, neighbors said, he had played impromptu games of baseball and football with other children, including a relative who went on to the National Football League.

But as an adult, the police said, his life took a much darker turn. He served five years in prison for an attempted murder in 2000. In November, the police said, he brandished a gun and hurled rocks at a nearby home before smashing the windshield of a parked car with a brick.

Little if any of that history was known to two New York officers — Brian Moore, 25, and Erik Jansen, 30 — as they patrolled in plain clothes. Watching for potential burglary suspects from their unmarked car, they saw a man adjust his waistband. They drove up to question him, but before they could step from their car, he turned and fired. A bullet struck Officer Moore in the face, passing through his cheek and critically wounding him.