One man was killed and another was hospitalized with serious injuries following a double stabbing inside a Brooklyn apartment building, police said Saturday.

The two victims were inside the Ave. D building near E. 31st St. in Flatbush at about 11:10 p.m. Friday when the bloody attack took place.

One victim, 66, was stabbed in the neck, and an unidentified man was stabbed in the left leg and lower back, police said.

Both men were rushed to Kings County Hospital, where the man stabbed in the leg died of his wounds. The 66-year-old victim was expected to survive.

Cops were trying to identify the dead man Saturday.

It was not immediately disclosed what sparked the stabbing or how the two victims knew each other.

No arrests have been made.

Cops were scouring the area for surveillance footage that can help them identify the stabber and track his movements.