DENVER (AP) — A shooting in downtown Denver in the United States (US) amid fans celebrating the Nuggets’ first NBA championship win was likely sparked by a drug deal gone wrong, police said Tuesday.

The violence left 10 people wounded, including one of two people arrested in connection with the shooting.

All of the injured — nine men, one woman — are expected to survive, including five or six people that police believe were bystanders not involved in the drug deal, Chief Ron Thomas said at a news conference. He said 20 rounds were fired at the scene, roughly a mile from Ball Arena where the Nuggets defeated the Miami Heat on Monday night.

A total of five handguns were found by investigators but testing still needs to be done to determine whether they were used at the shooting, the police chief said. A “significant quantity” of suspected fentanyl pills were also found at the scene, Commander Matt Clark said, along with cash.

Of the 10 people taken to the hospital, four underwent emergency surgery at the same time at Denver Health Medical Center. Five were still there on Tuesday afternoon, all in fair condition, said Dr Eric Campion, a trauma surgeon.

The Nuggets’ win drew thousands of people downtown and the shooting happened as the celebration was winding down after midnight, authorities said. Still, hundreds of police officers were massed in the area when the gunfire broke out.

Scott D’Angelo was livestreaming the celebrations when he heard several loud pops one after another, sparking pandemonium as people dove for cover or jumped over cement barricades. Police in riot gear ducked and drew their guns while yelling for people to find shelter.

Crouching on the ground, the 58-year-old said his arms were shaking with nerves and he felt an asthma attack coming. He heard a female voice not a dozen feet (three metres) away screaming in pain. Another victim lay just beyond the first, D’Angelo said, as officers rushed to provide care.

An overhead city surveillance video without audio released by police showed officers swarming toward the apparent scene of the shooting after gunshots were heard.

The firing stopped after roughly 20 seconds, D’Angelo said after consulting his footage, which he has handed over to investigators. As ambulances arrived, D’Angelo saw bullet casings only feet from where he’d dropped to the ground.

Authorities were still investigating how many people were involved in the shooting. Two men are being held on suspicion of being felons who are barred from having a firearm, said Clark, the police commander. Neither man had lawyers listed as representing them in court records yet.

One of the men ran from the scene despite being wounded and was arrested several blocks away with a handgun and fentanyl. The other was arrested in a car in a parking lot across the street from the shooting after police found a firearm hidden in its floorboards, Clark said. No one in the car was wounded, he said.

The gunfire broke out in downtown Denver’s LoDo district, which is known for its restaurants and nightlife. Yellow police tape had sealed off the area overnight Tuesday as investigators with flashlights scoured the scene, which was dotted with evidence markers and what appeared to be detritus left over from the celebrations, including an e-scooter and a green rental bike.

D’Angelo said he felt “kind of numb” after witnessing a mass shooting firsthand.

“To target somebody, and indiscriminately shoot innocent bystanders, even trying to think about it, it’s like — I have a huge emotional, a lot of feelings that I really can’t explain,” he said.

The shooting happened in the same area where fans celebrated the Colorado Avalanche hockey team winning the Stanley Cup last year without any serious problems. Thomas said police made similar preparations for the Nuggets’ possible championship.

“What we couldn’t have planned for was a drug deal right in the middle of a celebration,” Thomas said.