They're looking at the possibility a beige van or large SUV with a box-shaped rear was involved and would likely have a black paint transfer or scrape mark on the front, left bumper.
Birdsong and her 3-year-old son were hurt in the wreck. Birdsong's 16-year-old stepdaughter Kailee Nicole Birdsong was killed.
"There's no sign of them hitting the brakes," father Robby Birdsong said. "I don't understand why it was so important for them to cross the intersection that badly. I just really need to know why they didn't stop and check on them."
The child was in a safety seat, but police say it doesn't appear the driver or the teen were wearing seatbelts.
"People watching this just make sure you wear your seatbelts because she'd probably be alive if she had her seatbelt on," Birdsong said.
Anyone who witnessed the crash is asked to contact the Cary Police Department at (919) 469-4012.