A married father of two with a third child on the way, the 36-year-old was working a New York Police Department-approved private security detail, in uniform, when he and three other people were killed Monday at the Manhattan skyscraper that houses the NFL's headquarters and other corporate offices.

New York City police officers lined city streets to honor their colleague who was killed when a gunman entered a Midtown Manhattan office building and opened fire earlier this week.Officer Islam was one of four people killed inside the building on Monday evening.

Four people were killed and a fifth was injured in a mass shooting at a Midtown Manhattan office building on Monday. The injured victim was an employee at the NFL, according to NFL Commissioner Roger Goodell. Here's what we know about the four people killed: