Chants of "do something" broke out at a vigil honouring the nine people killed and 27 injured by a gunman who opened fire on revellers in Dayton, Ohio on Sunday.
The heckling - which also included the words "make a change" - began as Ohio governor Mike DeWine spoke at the gathering, attended by hundreds.
Masked gunman Connor Betts, 24, was armed with a .223-calibre rifle, capable of holding at least 100 rounds of ammunition, when he fired at people in the state's entertainment district.
Betts was gunned down within 30 seconds of the start of his rampage.
Among those killed in the attack was the sister of the gunman, 22-year-old Megan Betts.
Police identified the other victims as Monica Brickhouse, 39; Nicholas Cumer, 25; Derrick Fudge, 57; Thomas McNichols, 25; Lois Oglesby, 27; Saeed Saleh, 38; Logan Turner, 30; and Beatrice N Warren-Curtis, 36.
The gunman was white and six of the nine killed were black, police said.
The Ohio shooting came less than 24 hours after a man opened fire in a crowded El Paso, Texas, shopping area, leaving 20 dead and more than two dozen injured.
Responding to the shootings, US president Donald Trump tweeted: "Today, I authorized the lowering of the flags to half-staff at all Federal Government buildings in honor of the victims of the tragedies in El Paso, Texas, and Dayton, Ohio.
"The flags at the White House will be lowered today through Thursday, August 8. Melania and I are praying for all those impacted by this unspeakable act of evil!".
A number of anti-gun activists responded to the tweet, calling on the president to implement stricter gun laws in the country.
One Twitter user wrote: "Thank God you lowered the flags. I was worried nothing would be done about this gun violence epidemic."
Another posted: "Authorise a bad [sic] on guns and you won’t have to lower the flags, due to mass shootings."