BOULDER, Colo. — A shooting at a crowded Colorado supermarket that killed 10 people, including the first police officer to arrive, sent terrorized shoppers and workers scrambling for safety and stunned a state that has grieved several mass killings. A lone suspect was in custody, authorities said.

Hundreds of police officers from throughout the Denver metropolitan area responded to the Monday afternoon attack, converging on a King Soopers supermarket in a busy shopping plaza in southern Boulder. SWAT officers carrying ballistic shields slowly approached the store as others quickly escorted frightened people away from the building, some of its windows shattered. Customers and employees fled through a back loading dock to safety. Others took refuge in nearby shops.

One suspect was in custody, a tearful Boulder Police Chief Maris Herold said late Monday. Authorities didn’t identify the suspect, though Boulder County District Attorney Michael Dougherty said the suspect was the only person injured and was receiving medical care.

Officers had escorted a shirtless man in handcuffs, blood running down his leg, from the store during the siege. Authorities would not say if he was the suspect. Foothills Hospital in Boulder was treating one person from the shooting scene but refused further comment, said Rich Sheehan, spokesman for Boulder Community Health, which operates the hospital.

“This is a tragedy and a nightmare for Boulder County,” Dougherty said. “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.”

Herold identified the slain officer as Eric Talley, 51, who had been with Boulder police since 2010. He was the first to arrive after responding to a call about shots fired and someone carrying a rifle, she said.

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

Dozens of police and emergency vehicles, their lights flashing, escorted an ambulance carrying the officer from the shooting scene after nightfall. Some residents stood along the route, their arms raised in salute.

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

Dougherty said it was too early to speculate on a motive and that the investigation involving local, state and federal agencies would take days.

The attack in Boulder, about 25 miles (40 kilometers) northwest of Denver and home to the University of Colorado, stunned a state that has seen several mass shootings, including the 1999 Columbine High Schoolmassacre and the 2012 Aurora movie theater shooting.

Monday’s midafternoon attack was the seventh mass killing this year in the U.S., following the March 16 shooting that left eight people dead at three Atlanta-area massage businesses, according to a database compiled by The Associated Press, USA Today and Northeastern University.

It follows a lull in mass killings during the pandemic in 2020, which had the smallest number of such attacks in more than a decade, according to the database, which tracks mass killings defined as four or more dead, not including the shooter.

Rep. Joe Neguse, a Democrat whose district includes Boulder, said Tuesday on “CBS This Morning” that “enough is enough” when it comes to political impasses that keep gun control laws from passing Congress.

“The time for inaction is over. It does not have to be this way. There are commonsense gun legislation reform proposals that have been debated in Congress for far too long,” Neguse said. “The gun lobby and so many others have stopped the ability to make meaningful reforms in the past, but that’s no excuse. I think the American people are tired of excuses. So it’s time for us to roll up our sleeves in the Congress and muster the political will power to actually get something done.”

Dean Schiller said he had just left the supermarket when he heard gunshots. He saw three people lying face down — one in a doorway and two in the parking lot. Schiller said he couldn’t tell if they were breathing.

Sarah Moonshadow and her son, Nicolas Edwards, had just bought strawberries when they heard gunfire. Moonshadow told The Denver Post they ducked and “just ran.” Outside, Edwards said, arriving police pulled up next to a body in the parking lot.

“I knew we couldn’t do anything for the guy,” he said. “We had to go.”

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 heard at the beginning of the video.

Investigators had just started sorting through the crime scene and conducting witness interviews, Dougherty said. Matthew Kirsch, the acting U.S. 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.

White House press secretary Jen Psaki tweeted that President Joe Biden had been briefed on the shooting. Gov. Jared Polis, meanwhile, said in a statement that “Today we saw the face of evil. I am grieving with my community and all Coloradans.” The King Soopers chain said in a statement that it was offering prayers and support “to our associates, customers, and the first responders who so bravely responded to this tragic situation.”