The man who allegedly gunned down a 16-year-old girl in her Brooklyn apartment was in police custody Thursday night.

Cops identified 24-year-old Taariq Stephens as the man who killed Shemel Mercuis as she was baby-sitting Tuesday in East Flatbush.

Earlier on Thursday, cops released video of the suspect.

Stephens, armed with a “dangerous” .40-caliber Kel-Tec gun, can be seen in surveillance video racking the firearm in the hallway outside Mercurius’ door.

He was charged with murder at the 67th Precinct station house Thursday night.