The Veritas Project is a government-funded project investigating supernatural occurrences. The team consists of Nate and Sarah Springfield, and their twin children, Elisha and Elijah, a Christian home-schooling family, who use a Biblical approach to accomplish their missions.
There are only two books in the series, the first being Hangman's Curse, but with this particular series they don't necessarily have to be read in order.
Hangman's Curse:
Seemingly random students at a school become suddenly sick and begin acting oddly, losing their sanity and believing they see a ghost named Abel Frye. On all of the affected students' lockers, a hangman symbol is placed by a mysterious person or being. Elisha and Elijah Springfield enroll as students to investigate.