Hangman's Curse — Frank E. Peretti

Trouble is brewing in Baker, Washington. Three popular athletes have been stricken with a mysterious ailment--or phenomenon--that leaves them ranting incoherently, paranoid, and eventually comatose.
Ever ready to seek truth and justice, teen twins Elisha and Elijah and their parents hop in their van, the Holy Roller, and head to Washington. As members of a top-secret presidential investigative team, the Veritas Project, the family must be prepared to go undercover at any moment. Soon they are deeply embroiled in a deadly mystery that involves witchcraft, bullying, the ghost of a boy who hanged himself long ago, and a most diabolical weapon.