Investigators did not identify the targets on the list. They included courthouses and the garlic festival, Mr. Bennett said, and all of the organizations operated nationally. The list did not include any people, and the police have not recovered any manifesto.

The F.B.I.’s announcement came two days after its office in El Paso opened a domestic terrorism investigation into the gunman who killed 22 people at a Walmart. And later on Tuesday, the F.B.I. said it had uncovered evidence that the gunman who killed nine people, including his sister, in Dayton, Ohio, had been exploring “violent ideologies.”

While a federal statute defines domestic terrorism, it carries no penalties for people the government considers domestic terrorists, who are usually prosecuted under gun, hate crime and conspiracy laws.

The gunman in Gilroy wore a bullet-resistant vest as he used an AK-47-style rifle and fired 39 rounds at attendees of the annual garlic festival on July 28, Chief Scot Smithee of the Gilroy Police Department said on Tuesday. Officers found a 75-round drum magazine and five 40-round magazines near the gunman, who was 19 and who killed himself after being shot multiple times by the police, Chief Smithee said. Officers also found a shotgun in the gunman’s car and a backpack in a nearby creek with two more 40-round magazines.

The gunman’s parents said on Tuesday that they were “deeply shocked and horrified” by what their son had done.