Jack and Jill by James Patterson. 
In the nation's capital, homicide detective Alex Cross tracks a diabolical killer whose prey ranges from an ordinary little girl to the president. A child killer is stalking the inner city of Washington, D.C., his latest victim Shanelle Green, an adorable first grader from Sojourner Truth School. This killing is especially unsettling to Detective Alex Cross. Sojourner Truth is the school his son Damon attends, just four blocks from his home.   While the death of an inner-city black child doesn't garner much media attention, another murder is making big headlines. The same day that Shanelle was beaten to death, Senator Daniel Fitzpatrick was found handcuffed to a bed and shot execution style. The only clue the police have to go on is a bizarre rhyme, signed  Jack and Jill.