Nothing Left to Lose question

I looking for a similar book but I can't remember the title. I think the book begins with the girl walking/running in a park with earphones. Suddenly she saw something that she should have ( like something with a gang or a murder I don't remember ) Then she has to be put underproduction because of the thing she had seen because a gang's boss want to kill her or something like that. She meet her bodyguard (don't remember I think it is a cop.swat not sure at all) and they fall in love etc.. Does anybody know this book? ( I know there is not a lot of details) I have read it a few years ago. Thanks for your help !