Police said they have very little to go on, and only know that the vehicle was a dark-colored two-door car. It likely has extensive front-end damage.
Costlow was four months pregnant. She was rushed to a local hospital to be treated for severe head injuries, but she did not survive.
Her boyfriend, Aaron Desmond, who was with her at the time, said the suspect never even stopped. The two were riding bicycles when Costlow was struck from behind.
"I heard a big 'thud' behind me, the car slowed down, saw what it did, and it put its brights on me and left," Desmond said.
Desmond said the driver knew exactly what happened, but didn't care.
"Slowed down, looked at her, instantly blood everywhere, and then just took off," said Desmond.
"We absolutely need the help of the public in this case," said Gardena Police Lt. Mike Saffell. "There's not much suspect description about the vehicle, so anyone that knows anything about this collision, please call."
Police have posted a neon sign at the intersection asking for the public's help. Anyone with information is urged to call the Gardena Police Department at (310) 217-6135.