She Said/She Saw by Norah McClintock

Tegan was in the backseat of Clark's SUV when someone walked up and shot both Clark and Marvin, both in the front. She wasn't looking at the shooter, she was looking at Marvin, so when the police ask her for a description of the shooter, she can't tell them anything.

But no one believes she didn't see anything. Not her friends, who are shunning her, not her family, who want her to tell the truth, and not Clark and Marvin's families, who are convinced she's scared for her own life and can't be concerned with her two best friends.

Kelly, Tegan's sister, believes Tegan at first. It's entirely possible she didn't see anything. Then she finds out that Marvin, who had a crush on Kelly, told Tegan. And Tegan convinced Marvin to get drugs for the party they were at, even though he had gone clean. Not sure what to believe, Kelly also shuns Tegan.

Until Tegan decides the only way to tell the truth is to flush out the killer herself.

The story is told from Tegan and Kelly's points of view and could have been a much longer book. I found the concept pretty amazing, and enjoy the way McClintock builds suspense and doubt. Both main characters are likable but have their moments.