El Paso police chief Greg Allen said the rifle used in the shooting was purchased legally.
"It seems to just defy believability he would shoot his own sister, but it's also hard to believe that he didn't recognize it was his sister, so we just don't know", said Dayton Police Chief Richard Biehl. "To have that level of weaponry in a civilian environment, unregulated, is problematic".
Betts was killed by police officers on patrol nearby 30 seconds after he opened fire. Special Agent Todd Wickerham did not specifically say what the FBI was investigating, but said the agency is looking into who might have helped Betts, and why he chose the specific target of Dayton's OR entertainment district.
Nine people died and 27 others were injured in the shooting.
Surveillance video released by the police department shows Betts struggle as he turns a corner and attempts to enter a bar where dozens of people sought refuge from him.
Betts fatally shot one person in an alleyway before turning his fire on his sister and their friend, police have said.
Biehl said Betts' parents were having a tough time.
Betts was gunned down within 30 seconds of the start of his rampage.
But the devastation was already done. Officials have recovered 41 spent casings, but added suspect may have fired more rounds.
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"It's baffling and disgusting that somebody who's been talking for 10 years about wanting to shoot people could easily, so easily, get access to a military grade weapon and that much ammo", said Hannah Shows, a former high classmate who remembered seeing Betts look at people and imitate shooting at them. The file included a 2015 crash report from when Betts ran his auto into a ditch and a copy of a 2012 OH state law that requires all sealed records of juvenile crimes to be expunged either after 5 years or on the offender's 23rd birthday.
Among the states that ban high-capacity magazines, some prohibit magazines that hold more than 10 rounds, and others ban magazines that hold more than 15 rounds. The three apparently parted at some point before the shooting.
"This is a nightmare for them and I think they are struggling as I think you can understand", he said according to Reuters.
OH hospital officials say 37 people have been treated for injuries connected to the mass shooting in Dayton over the weekend. Talking about serial killers made sense as it was a theme in a Sinclair Community College psychology class they both were taking, a captivation with disaster and violence was offset by the sweetness of a "perfect gentleman", and joking about a desire to hurt others was seen as the coping tool of a man grappling with illness. Numerous victims were shot at a Walmart. Sources reported that police were in search of the suspect's house. "Just based on where we're at now, we are not seeing any indication of race being a motive". Former classmates of Betts told The Associated Press that Betts had once been suspended from Bellbrook High School for writing a "hit list" in the school's bathroom.
Betts' rampage was the second mass shooting in the US over the weekend, both leaving a total of 31 people dead and more than 50 injured.
Police did not detail the Betts siblings' relationship.
Biehl said the male companion is cooperating with authorities but officials still don't know the exact nature of his relationship with Emily. Crusius, who was taken into custody, told authorities that he wanted to kill as many Mexicans as possible.
The death toll from the two weekend shootings rose to at least 31 on Monday.
"We have a lot of evidence still to go through".