The FBI has opened a domestic terrorism investigation into the mass shooting at a California food festival after it discovered a “target list” compiled by the gunman whose relatives apologized Tuesday and said they were “horrified” by his actions.

The FBI disclosure came during a funeral mass for Keyla Salazar, a 13-year-old middle schooler who was one of three people killed on July 28 by gunman Santino William Legan during the Gilroy Garlic Festival. Thirteen others were injured.

It was the first of three mass shootings within a week that killed a total of 34 people in Gilroy, Dayton, Ohio, and El Paso, Texas. The attacks have prompted widespread calls for gun reform and heightened mental health care.

Read more: AP