The New York police officer killed in a mass shooting in a Manhattan skyscraper on Monday was an immigrant who left behind a pregnant wife, two young sons and distraught parents, while friends and officials in Bangladesh and the United States described him as a hero.

Investigators are examining why the gunman who killed four people before taking his own life at a New York City office tower targeted the National Football League and whether he suffered from a brain disease.

There were 23 shell casings and 13 bullet fragments recovered from the lobby of 345 Park Avenue and 24 spent shell casings on the 33rd floor, police said.