Colorado hit by another mass killing

A man seen bloodied and limping as he was led away in handcuffs has been arrested on suspicion of killing 10 people, including a police officer, at a Colorado supermarket, marking America's second fatal mass shooting in a week.

It was also the latest shooting tragedy to hit a state that was the site of two of the most infamous mass murders in US history.

Witnesses at King Soopers in Boulder county, 50 kilometers northwest of the state capital Denver, said they initially heard multiple loud bangs outside the shop.

Nevin Sloan, who narrowly escaped with his wife Quinlan, said panic then set in as the sound of gunshots drew closer and shoppers were forced to decide whether to stay put or make a break for the exit.

"All of a sudden we started hearing more, like 'bang, bang, bang, bang' and I sprinted over to her and just told her 'hey we need to get out of here,' " he said.

Police said among the 10 people killed was officer Eric Talley, 51, who was first on the scene. "Didn't surprise me he was the first one there," his father said, adding his son "loved his family more than anything."

Police believed the suspect, armed with a rifle, acted alone. They offered no possible motive.

Live-streamed video earlier showed a white middle-aged man - shirtless and with a bloodied leg - detained by police. A suspect was the only person suffering "serious injuries at this point," police said.

Dozens of armored vehicles, ambulances and armed personnel including FBI agents and SWAT teams were deployed to the scene. The incident follows another mass shooting last week at Asian-owned spas in the state of Georgia that left eight dead.

Colorado has previously suffered two of the most infamous mass shootings in US history - massacres that prompted nationwide soul-searching but did not result in major changes to the country's lax gun ownership laws.

In 1999, two boys shot and killed 12 classmates and a teacher at Columbine High School.

Then in 2012, a heavily armed man stormed a movie theater in Aurora, murdering 12. The gunman is now serving a life sentence without the possibility of parole.