The FBI on Tuesday opened a probe into the mass shooting that left nine dead in Dayton, Ohio, over the weekend.

"We have uncovered evidence throughout the course of our investigation that the shooter was exploring violent ideologies, and based upon this evidence, we're initiating an FBI investigation side by side with the Dayton Police Department," Todd Wickerham, the special agent in charge of the FBI's Cincinnati field office, said at a news conference.

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Wickerham said the investigation will focus on figuring out what ideologies influenced 24-year-old Connor Betts, who might have helped Betts before the attack and why he chose the specific target of Dayton's Oregon entertainment district.

Wickerham later added that no evidence has been found to suggest the shooting was motivated by race or a desire from Betts to kill his sister, who was one of the nine victims.

Richard Biehl, Dayton's police chief, stressed that there is no "true clarity of motive of the assailant" yet.

"Materials reviewed thus far reveal the individual had a history of obsession with violent ideations to include mass shootings and had expressed a desire to commit a mass shooting," Biehl said during the news conference.

"Subsequent material has revealed an orientation toward violent ideologies which elevate this case to one of federal interest," he added.

The shooting in Dayton, in which more than two dozen people were injured, came less than 24 hours after another mass shooting in El Paso, Texas, that left 22 people dead. The shootings are not believed to be linked.

Also on Tuesday, the FBI announced it is opening a domestic terror probe into last week's mass shooting in Gilroy, Calif. The alleged shooter in Gilroy had a "target list," the FBI said, including religious groups and "political organizations from both major political parties."