Under The Blood (en Inglés) - Jim Toner

Sixteen-year-old Gil Leduc walks to school one day when he stumbles across the dead body of his close friend Paul Connors. Paul had once been an active member of the youth group at Niagara Pentecostal Assembly Church before a few misfortunes in his life. Paul joined the Black Ravens, a Satanic gang known to hold rituals in an abandoned house in the Love Canal area of Niagara Falls, New York. 

Gil vows to find the killer, but Niagara Falls Detective Patricia Daulton, a hard-nosed by the book cop, tells him not to interfere with the investigation. 

Gil receives numerous threats on his life. Someone even calls him claiming to be Satan.

Not only do people threaten Gil, but they also threaten his girlfriend Jan Barrio and her family. Jan's father Pablo, a Niagara Falls Police officer, urges Gil to leave the investigating to the police.

Under the Blood is a young adult mystery combining the suspense of a Hardy Boys novel and the spiritual warfare elements of a Frank Peretti novel.