A shooting incident at a Colorado supermarket has left 10 people dead, including a police officer who was the first to respond to the scene, authorities said on Tuesday.

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

Investigators had just begun sorting through evidence and witness interviews and didn’t have details on a motive for the shooting at the King Soopers store in Boulder, which is about 25 miles (40 kilometers) northwest of Denver and home to the University of Colorado, said Boulder County District Attorney Michael Dougherty.

“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.”

The 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 Northeastern University and two leading news organizations based in the U.S.

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.

The slain officer was identified as Eric Talley, 51, who had been with Boulder police since 2010, Herold said. He went to the store after 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 of Talley.

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