A man tied up the father of a 15-year-old girl and then assaulted the teen early Wednesday morning during a home invasion and robbery on South Interstate 35, Austin police said.

Detectives say the teen heard a knock on her apartment door before 5 a.m. Her stepfather opened the door, and was confronted by a light-skinned man with a gun.

That's when he tied up the father and sexually assaulted the 15-year-old in her bedroom.

The father managed untie himself and run outside to flag down help.

The attacker ran from the apartment after stealing their money. He's described as being in his mid-20's, 5'7" to 5'9" tall, with a medium build and short dark hair. He was carrying a small black handgun.

Police don't believe the suspect knew his victims, and they believe he was looking for another apartment. Officers were canvassing the neighborhood, looking for leads Wednesday.

If you have any information, call the Child Abuse Unit at974-6880 or CrimeStoppers at 472-TIPS.