A man is on the loose more than a month after attacking a woman inside a Brooklyn subway station, police said late Monday evening.

The man, who is 20 to 30 years old, went ballistic after the 63-year-old woman bumped into him on the stairwell of the Utica Avenue station on July 23, police said. He immediately slugged her in the back of the head at about 3 p.m. before threatening her.

"Leave me alone," police said he yelled at her. "If you follow me, I'll kill you."

The woman was taken to Kings County Hospital with swelling to the back of her head, police said.