New York City is mourning NYPD Officer Didarul Islam and three other people killed in Monday's shooting inside an office building at 345 Park Ave.

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.

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.