A mass shooting at a Colorado supermarket saw ten people killed, including a police officer who was the first to respond to the scene.

Police arrested a suspect, but did not reveal his name or any details about the shooting at a news conference where Boulder police Chief Maris Herold fought back tears.

Investigators have started sorting through evidence and witness interviews and do not yet have details on a motive for the shooting at the King Soopers store in Boulder, which is about 25 miles northwest of Denver and home to the University of Colorado, said Boulder County District Attorney Michael Dougherty.

He said: “This is a tragedy and a nightmare for Boulder County.

“These were people going about their day, doing their shopping. I promise the victims and the people of the state of Colorado that we will secure justice.”

The attack was the seventh mass killing this year in the US following last week’s shooting that left eight people dead at three Atlanta-area massage businesses.

The killed officer was identified as Eric Talley, 51, who had been with Boulder police since 2010, Mr Herold said.

“He was by all accounts one of the outstanding officers of the Boulder Police Department and his life was cut too short,” Mr Dougherty said.

Identities of the other nine victims were not disclosed on Monday night as police were still notifying their family members.

Matthew Kirsch, the acting US attorney for Colorado, pledged that “the full weight of federal law enforcement” will support the investigation.

He said investigators from the Bureau of Alcohol, Tobacco, Firearms and Explosives were at the crime scene, along with FBI agents.

Officers had escorted a shirtless man with blood running down his leg out of the store in handcuffs but authorities would not say if he was the suspect.

They did say the suspect was receiving medical care and was the only person injured who did not die.

Dean Schiller told The Associated Press that he had just left the supermarket when he heard gunshots and saw three people lying face down, two in the car park and one near the doorway.

He said he “couldn’t tell if they were breathing”.

Video posted on YouTube showed one person on the floor inside the store and two more outside on the ground. What sounds like two gunshots are also heard at the beginning of the video.

Law enforcement vehicles and officers massed outside the store, including Swat teams, and at least three helicopters landed on the roof.

At one point, authorities said over a loudspeaker that the building was surrounded and that “you need to surrender”.

Colorado Governor Jared Polis tweeted a statement that his “heart is breaking as we watch this unspeakable event unfold in our Boulder community”.