A Colorado shooting in one supermarket has claimed the lives of 10 people, including a police officer who was the first responder to the scene, according to officials.

Police have already captured the shooter but did not reveal the suspect's name and details regarding the shooting, according to an Associated Press report.

Officials said it would take days to investigate the large crime scene and to notify the families of those that had been killed.

This was the latest mass killing this year in the United States after the March 16 Atlanta spa shooting, which left eight people dead, including some Asian women.

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The Colorado Shooting Incident

One civilian, Dean Schiller, said he had just left the King Soopers supermarket in Boulder, Colorado on Monday afternoon, when he heard gunshots and saw three people lying on the ground, face down.

Two of the victims were in the parking lot, and one was near the doorway. Schiller said he could not tell at the time if they were breathing.

Law enforcement vehicles and officers gathered outside the store, including SWAT teams. And at least three helicopters were on the supermarket's roof.

Civilian James Bentz told the Post that he was in the meat section when he heard what he thought was a misfire, followed by a series of gunshots.

"I was then at the front of a stampede," he said as CNBC reported.

Colorado Governor Jared Polis addressed the incident through a tweet, saying his heart is breaking as he watched the event unfold in their community.

Slain Police Officer

The deceased police officer was Eric Talley, 51, who had been serving the Boulder police since 2010, according to Boulder police Chief Maris Herold.

Boulder County District Attorney Michael Dougherty described the Colorado shooting incident as a nightmare for Boulder County.

"I promise the victims and the people of the state of Colorado that we will secure justice," Dougherty said in the AP report.

Meanwhile, the identity of the other nine victims has yet to be released as police were still notifying their families.

Matthew Kirsch, the acting U.S. attorney for Colorado, promised that the full weight of federal law enforcement would be imposed upon the investigation.

Bureau of Alcohol, Tobacco, Firearms and Explosives agents arrived on the scene, along with Federal Bureau of Investigation agents. 

U.S. Mass Shootings

Two major mass shootings were reported within a week of each other. Despite the frightening occurrence, experts said that shootings that killed at least 50 people and injured 200 more have been less common during the pandemic, Business Insider reported.

However, the Gun Violence Archive discovered that more private shootings significantly increased, with four or more people being shot by one person.

The archive noted that there were 611 mass shootings in 2020, and the majority of the suspects knew the victims.

Jillian Peterson, an associate professor of criminal justice at Hamline University, told the New York Times that it is likely the focus on the pandemic meant less talk of mass shootings for a gunman to become interested.

"There had been a hope that maybe we broke the cycle and maybe we won't return. Now that it's back, a number of scholars are really concerned," Peterson said in the report.

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WATCH: Reporters Describe What it Was Like Inside Supermarket in Colorado Shooting - From CNBC Television