PHILADELPHIA (CBS) — Police are investigating a home invasion and shooting in Northeast Philadelphia.

It happened around 2:30 a.m. Saturday in the 4800 block of Longshore Avenue in the city’s Tacony neighborhood.

Police say a 33-year-old man answered a knock at the door of his second floor apartment and found two armed men wearing black masks. Authorities say the man tried to shut the door, but the gunmen forced their way inside where they opened fire, striking the man his shoulder.

He was taken to Aria-Frankford Hospital in critical condition.

Two other people who were inside the apartment at the time of the shooting were not harmed.

Police say nothing was taken from the apartment.

Investigators say the motive for the shooting is unknown.