In the second mass shooting in less than 15 hours, at least nine people were killed and 27 injured early on Sunday after a gunman in body armor and wielding an AK-47-style assault rifle opened fire in downtown Dayton, Ohio, according to police.
Law enforcement officials tell ABC News the suspected gunman's 22-year-old sister is among those where were shot to death.
The suspected shooter was shot and killed by responding officers "in less than a minute" after opening fire in the bustling Oregon District of Dayton, Mayor Nan Whaley said Sunday morning on ABC's "This Week." Police said they were only aware of one shooter.
"What's scary about that is that if they were not there, the hundreds of lost lives we would've had in the Oregon District," Whaley said.
The suspected gunman was identified as 24-year-old Connor Stephen Betts, Lt. Col. Matt Carper, the assistant Dayton police cheif, said at a news conference Sunday afternoon.
Carper also released the names of all nine victims killed, including Betts' sister, Megan Betts. The victims ranged in age from 25 to 39.
Betts lived in the Dayton suburb of Bellbrook, where FBI and Dayton police executed a search warrant on his home Sunday morning, according to Bellbrook Police Chief Doug Doherty. The chief told reporters that his agency had no previous contact with Betts.
The mass shooting in Dayton followed a mass shooting Saturday morning in El Paso, Texas, where 20 people were killed and more than two dozen injured when a gunman, also wearing body armor and armed with an assault rifle, opened fire without warning.
The back-to-back massacres came a week after a gunman killed three people and injured more than a dozen others at the Gilroy Garlic Festival in Northern California.
Authorities responded quickly to the Dayton shooting, first reported at 1:07 a.m., and said police are regularly present in the crowded neighborhood full of bars and restaurants.
Whaley said the shooter wore armor and was carrying a .223-caliber rifle similar to an AK-47 and additional high-capacity magazines.
The shooting could have been much worse, according to Whaley, as "thousands of people" were in the bustling Oregon District at the time "enjoying their Saturday evening."
Miami Valley Hospital emergency physician Dr. Randy Marriott said at a news conference Sunday morning that his emergency room received 16 victims from the shooting in the Oregon District. He said four victims were admitted to the hospital, and one was in critical condition.
Marriott said the majority of victims his staff treated suffered gunshots wounds and several patients were injured running from the gunfire. He added that hospital staff received virtually no warning before wounded victims began arriving at the hospital in police cars and ambulances.
The Kettering Health Network, comprised of nine hospitals in Dayton, said multiple victims were also treated at its medical centers. Elizabeth Long, a spokeswoman for the hospital, said nine of the victims were treated at the network's Grandview Medical Center, where three remained in serious condition and three others were in fair condition.
Long said victims were shot in the lower extremities, and some suffered abdominal wounds. She said one victim suffered a laceration to the foot, running from the gunfire.
Asked if she suspects the gunman acted as a copycat of the mass shooting on Saturday morning in El Paso, Texas, that left 20 people dead and at least 26 injured, Mayor Whale said, "I can't speculate on that, frankly. We'll have more to say on that in the coming hours."
The shooting took place in Dayton's Oregon District, a collection of bars, restaurants and local businesses in the city's downtown. The entire shooting took place outside, Biele said.
Witnesses said the shooting erupted as hundreds of people were lined up outside on the sidewalk to get into nightclubs, or crowded around street musicians performing.
"Most of us can't get to our cars because there are bodies scattered all over across the street from our cars," an eyewitness told Dayton ABC affiliate WKEF. "People that were shot, hit, innocent people - we can't get home to our families now, and those people aren't going home to their families either."
Whaley said she received several calls from across the country.
"I'm heartbroken," Whaley tweeted Sunday morning. "Thank you to our first responders for all that you've done. We will share updates as we have more information."
In addition to local police, the FBI is on scene assisting with the investigation.
The Dayton Police Department said in a tweet just after 3 a.m. that they were "actively investigating an active shooter incident in the #OregonDistrict. Please avoid the area."
"This is a large scene and investigation. Thank you for your patience," Dayton police added in a subsequent tweet.
Two bars in the Oregon District - Hole in the Wall and Ned Peppers - wrote on Instagram that their staff was safe.
Kettering Health Network, which operates eight hospitals in the Dayton and Cincinnati areas, said it was treating patients, but offered no specifics.
The shooting comes just 14 hours after a gunman killed 20 people and injured over two dozen more at a Walmart 1,600 miles away in El Paso, Texas. The shooting was the eighth-deadliest in modern U.S. history. Five of the top 10 deadliest shootings in U.S. history have occurred since 2016.
President Donald Trump tweeted Sunday morning that the "FBI, local and state law enforcement are working together in El Paso and in Dayton, Ohio," adding that "updates will be given" as more information is gathered in both shootings.
He added in a subsequent tweet, "God bless the people of El Paso Texas. God bless the people of Dayton, Ohio."
During his Sunday mass, the pope referenced the three shootings in Gilroy, El Paso and Dayton, asking worshipers "to join my prayer for the people who lost their lives, the injured and their family members."
"I am spiritually close to the victims of the episodes of violence that have bloodied Texas, California, and Ohio in the United States, striking defenseless people," Pope Francis said.
ABC News' Josh Margolin and Aaron Katersky contributed to this report.
This is a developing story. Please check back for updates.