Protesters' chants of "do something" interrupted remarks from Ohio's governor during a vigil for the victims of a mass shooting on Sunday.
Republican Governor Mike DeWine was speaking in Dayton, where earlier 24-year-old Connor Betts shot and killed his sister and eight other people.
At least 27 people were injured in the shooting, the second in the US within 24 hours.
The governor told reporters he is "open to discussion" on gun reform.
On Monday, Dayton police chief Richard Biehl said officials were investigating whether the gunman intentionally targeted his sister, but still had not determined a motive.
"It just seems to defy believability that he would shoot his own sister, but it is also hard to believe he didn't recognise his sister, so we just don't know."
He added that Betts had modified his gun and used magazines that could have carried 250 rounds, though it was unclear whether they were full.
"That level of weaponry in a civilian environment, unregulated, is problematic," the police chief said.
During the vigil on Sunday, the state's governor was forced to speak in between the crowd's chants.
"We know that we cannot ease the pain of those families who have lost someone," Mr DeWine said. "We also know that we want to do something."
Dayton's Democratic Mayor Nan Whaley later asked the crowd to be more respectful, saying: "This is a vigil for the people that we've lost. There will be time to take action, but let us come together as a community as we work to heal."
Sunday's attack lasted less than 30 seconds, police say. Officers engaged and killed the gunman as he tried to make his way into a crowded bar.
Mr Biehl said that had he made it through the door, the loss of life would have been "catastrophic".
Following the attack, Mr DeWine - who ran as a pro-gun rights candidate last year - said "everything's on the table" regarding gun control policies, Cleveland.com reported, as long as the measures were constitutional.
"What can we do different that maybe we should do that will not eliminate the chance of something like this - I don't know if you can eliminate the chance of something like this - but rather to lessen the chance that it would occur?"
The governor this year proposed "red flag" legislation that would allow law enforcement to confiscate guns from those determined to be a threat to themselves or others.
The gunman fired his first shot at 01:05 local time (05:05 GMT) in the Oregon district of the city centre, Mr Biehl said. Dozens more rounds followed.
Security camera footage shows dozens of people racing through the doorway of the local Ned Peppers nightclub.
Seconds later, Betts, 24, is seen running towards the venue and being hit by police gunfire as he reaches the door.
He wore body armour and came carrying extra ammunition for his .223-calibre assault rifle with high-capacity magazines.
"Had this individual made it through the doorway of Ned Peppers with that level of weaponry, there would have been catastrophic injury and loss of life," Mr Biehl said.
The rifle was ordered online from Texas, police said, and there was nothing in his history that would have stopped him from buying the gun legally.
The gunman's sister, Megan Betts, aged 22, was among the dead.
"She was not the first victim, but she was one of the initial victims," Mr Biehl said.
Responding to questions about a possible racial element to the killings, Mr Biehl said there was nothing to suggest a "bias motive".
According to US media, Betts was suspended in high school for keeping a list of people he wanted to kill.
On Monday, Mr Biehl said police were looking into any available information, but that he was "reluctant" to interpret decade-old evidence as "indicative of what happened yesterday".
Nearby hospitals had received 27 people for treatment, and discharged 15 of them, by 10:00 local time.
The Gun Violence Archive, which categorises mass shootings in the US as four or more people shot or killed, says there have been 251 so far this year.
Who are the victims?
Police listed all nine of those who died. They are:
- Lois Oglesby, black female, 27
- Megan Betts, white female, 22
- Saeed Saleh, black male, 38
- Derrick Fudge, black male, 57
- Logan Turner, white male, 30
- Nicholas Cummer, white male, 25
- Thomas McNichols, black male, 25
- Beatrice Warren Curtice, black female, 36
- Monica Brickhouse, black female, 39
Who was the attacker?
Hours after he had been named by the media, police officials confirmed the gunman's identity as 24-year-old Connor Betts from Bellbrook, Ohio.
Police searched his home on Sunday.
Earlier, US media reported that his sister had been found dead in a car with her boyfriend. But police said the male she was shot alongside was a "companion of the suspect" and did not elaborate on the relationships.
"We don't know the thoughts of the shooter," Mayor Nan Whaley said.
FBI agents are assisting police with the investigation.
What's been the reaction?
At a series of news conferences, Mayor Whaley asked people to think of those people "who went downtown on a Saturday night and thought they'd be safe".
She said that the Oregon district was "one of the safest places in the whole region".
"Frankly, we're at a situation now in our country that these are so random," she said.
She also said the city had gone through a "really tough year", drawing a comparison to a series of 14 tornadoes that hit Dayton in March.
"What really goes through my mind is one seems completely preventable," she said. "I just question when is enough, enough."
President Donald Trump told reporters that "hate has no place in our country" shortly before boarding Air Force One in Morristown, New Jersey.
Mr Trump said mass shootings in the US had been "going on for years" and had to be stopped.
When asked about possible gun regulations, the president said "perhaps more has to be done" to prevent shootings, but stopped short of making any commitments.
On Twitter, local-born singer Lizzo called on people not to "normalise" shootings, saying that her family in Dayton were safe "but that's not the case for nine other families".
Just got off the phone w/ fam in Dayton... it was a close call for them but that’s not the case for 9 other families— |L I Z Z O| (@lizzo) August 4, 2019
between this & the terrorist attack in El Paso & recent other shootings I feel completely helpless.. make noise & bring awareness.. vote.. don’t normalize this
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