'We will secure justice.' Colorado reels from mass shooting.

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.

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 mid-afternoon attack was the seventh mass killing this year in the United States, 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.

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 from the scene receiving medical care.

Officers had escorted a shirtless man in handcuffs 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 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,” Ms. 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.

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

Investigators have just started sorting through the crime scene and conducting witness interviews, Ms. 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.”

The story was reported by The Associated Press. AP writers Colleen Slevin and Jim Anderson in Denver contributed.