A 15-year-old Woodbridge girl said she was sexually assaulted Monday by a man who came to her door and threatened her unless she let him in, Prince William County police said.

The same man had come to her door earlier and claimed to work for a cable provider, the police said.

Once inside the house, he showed what appeared to be a knife and assaulted her, police said. When she began to scream and resist, he fled.

Police said he was described as a black man in his 20s with a medium build who was last seen wearing a gray jacket, red shirt, black pants and black dress shoes.