(Radio Iowa) – A published report now says the suspected gunman in the murders of a Cedar Falls couple and their six-year-old daughter last month used a so-called ghost gun. The Des Moines Register quotes the parents of the 23-year-old from Nebraska as saying he ordered the parts for the untraceable weapon online and built it himself at home. The Register says the parents wrote in a letter to the paper that their son wanted the gun as protection for his family from a rising threat of violent crime.

Investigators say he used the gun to kill Tyler and Sarah Schmidt and their girl, Lula, in their tent at Maquoketa Caves State Park, and later turned the gun on himself. No motive is known and there’s no known link between the alleged gunman and the Schmidts. Their nine-year-old boy, Arlo, survived the shootings.