The 9th Judgment by James Patterson

The 9th Judgment is the title for the ninth book in the Women's Murder Club series featuring Lindsay Boxer by New York Times Bestselling author James Patterson and was released in 2010.
SFPD sergeant Lindsay Boxer and her squad are tracking down a man who approaches young mothers in shopping center parking garages and kills them and their infant children. This happens several times and causes widespread panic. The man then makes contact and demands a $2 million ransom in exchange for stopping the killing. He strikes at such random places and times that, despite Lindsay Boxer's protests, the police decide to pay the ransom and Boxer is selected to deliver the ransom and has the man guide her via cellphone. After forcing Boxer to strip to her panties to prove she is not wearing a wire, the man has her drive throughout San Francisco. The trek culminates on the Golden Gate Bridge where Boxer drops the suitcase with the ransom in it to a boat waiting below. The Coast Guard is able to apprehend the man in the boat, who turns out to be uninvolved in the killings—he had been told his picking up the suitcase was for a movie.
Upset at having not gotten his ransom, the man kills another mother and child. While at that crime scene, however, he was noticed by the owner of a store on Fisherman's Wharf. The owner recognized him as having bought a prepaid cellphone from his store earlier and gives Boxer and her team the surveillance tape from his store. From the tape, Boxer and her team identify the man and begin tracking him down... (Summary by Wikipedia)