BOULDER, Colorado (AP) – 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 did not 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,” Mr. 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.”

Ms. 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.

Mr. 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 northwest of Denver and home to the University of Colorado, stunned a state that has seen several mass shootings, including the 1999 Columbine High School massacre 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.

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. Mr. Schiller said he could not 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, Mr. 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.”