The FBI has opened a domestic terrorism investigation into the California food festival shooting that left three people dead, including two children.
Officials revealed they had discovered gunman Santino William Legan's "target list" which included religious and federal buildings.
Agent John Bennett said Legan, 19, was "exploring violent ideologies" but the FBI has not yet confirmed a motive.
Police are also linking two other recent shootings to violent ideology.
An attack in El Paso on Saturday which left 22 people dead is being investigated as a domestic terrorism case. Law enforcement officials are studying an anti-Hispanic document posted online prior to the attack.
And on Tuesday, the FBI said a gunman in Dayton, Ohio, where nine people were killed on Sunday, also followed a "violent ideology". But they said they had found no evidence that the attack was racially motivated.