LINCOLN, Neb. (AP) — Lincoln police say a group of four teenagers is responsible for more than 30 car thefts during the past year.

Two of the boys have been locked up in the Youth Detention Center and two others have been cited.

Officer Chris Fields says a 15-year-old boy started stealing cars in summer 2013 and that his friends later joined him. He says the group was involved in at least two hit-and-run crashes. Fields says they torched a 2001 Isuzu Trooper by setting off an artillery shell under the passenger seat.

The Lancaster County Attorney’s Office has filed petitions against three of the boys in juvenile court.

Their names haven’t been released.