FBI agent Todd Wickerham told a news conference two days after the massacre in Dayton's historic downtown Oregon District that investigators have yet to conclude what motivated the killer or whether he may have had an accomplice.
Police said in the initial aftermath of Sunday morning's bloodshed that they believed the slain suspect, identified as 24-year-old Connor Betts, a white man from Bellbrook, Ohio, had acted alone.
The suspect, who was wearing body armor and a mask, opened fire with an assault-style rifle fitted with a high-capacity ammunition drum that could hold 100 rounds, police said. Authorities said officers on routine patrol in the area arrived on the scene and shot the gunman dead 30 seconds after the violence began.
In addition to the nine people, including Betts' sister, who were killed, more than two dozen others were injured in the attack, which came 13 hours after a shooting spree that claimed 22 lives in El Paso, Texas.
NO TIES FOUND TO TEXAS CARNAGE
The assailant in Texas surrendered to police and has been charged with capital murder in what authorities are treating as a hate crime and an act of domestic terrorism.
Federal agents have found no clues suggesting the Dayton gunman was influenced by the rampage in El Paso, said Wickerham, the agent in charge of the FBI's Cincinnati office.
However, he said, "the individual had a history of obsession with violent ideations, including mass shootings, and expressed a desire to commit a mass shooting."
"We have uncovered evidence throughout the course of our investigation that the shooter was exploring violent ideologies," Wickerham added.
Nevertheless, much remained unclear. The agent said investigators were still seeking to determine what particular ideology may have influenced the gunman, "who, if anyone, helped him or had any advance knowledge of his intentions to conduct this attack, and why he committed this specific act of violence."
Wickerham also echoed previous statements by the city's police chief, Richard Biehl, that there was no indication so far that the shooting spree was racially motivated, even though six of the nine dead were African-American.
Authorities have previously said Betts was known to have been a troubled youth in high school, at one point drawing up a "hit list" of students he wanted to kill or otherwise harm.
The investigation update came as Ohio Governor Mike DeWine, responding to mounting public pressure for action to curb gun violence, proposed a "red flag" law that would allow a judge to order firearms confiscated from any individual deemed to be a threat to themselves or others.
"We have an obligation to each other," DeWine, a Republican backed by the National Rifle Association gun lobby, said at a news briefing. "If someone is showing signs of trouble or problems, we must help and we must not turn away."
The governor was heckled on Sunday night as he spoke at a vigil for the victims of the rampage. Protesters repeatedly chanted "Do something!," a reference to perceived state and federal inaction to curb U.S. gun violence."Some chanted 'Do something!' and they're absolutely right," DeWine said on Tuesday. "We must do something, and that is exactly what we're going to do."