Ten people were killed Monday, including a police officer, when a gunman opened fire at a supermarket in Boulder, Colorado, the second mass shooting in the U.S. in a week.

“These were people going about their day, doing their shopping,” Boulder District Attorney Michael Dougherty said at a late-night press briefing. He described the day’s events as “a tragedy and a nightmare.”

“My heart goes out to the victims of this incident and I am grateful to the police officers that responded,” police chief Maris Herold said.

Boulder Police Commander Kerry Yamaguchi said a suspect has been arrested. “There is no ongoing public threat,” Yamaguchi said. “We do have a person of interest in custody. That person was injured during the incident and is being treated for the injuries.”

No motive for the attack was disclosed. Matthew T. Kirsch, the First Assistant United States Attorney for the District of Colorado, said the FBI and other federal agents will be assisting in the case. He promised that the “full weight of federal law enforcement” will be brought in the investigation.

The shooting comes six days after eight people – six of them women of Asian descent – were killed by a gunman at three spas in Atlanta and a nearby county.

Roberto and Ilce Rivero’s son, who attends Fairview High School near the King Soopers feels shootings are the norm, and he hopes that feeling will change.

“I hope this tragedy… will help change laws, gun laws in particular, and we can all work together to make a better, more peaceful world for our children,” Roberto said, then adding, “This is a sad day for Boulder.”

The White House said President Joe Biden has been briefed on Monday’s attack and will be kept up to date on developments. The FBI office in Denver tweeted that it’s assisting in the investigation at the request of the Boulder police.

Officers armed with tactical gear and rifles responded Monday afternoon to reports of an active shooter at a King Soopers in the southern part of Boulder, about 25 miles northwest of Denver.

At 2.49 p.m. local time, the Boulder Police Department tweeted an alert for people to stay away from the area around the supermarket.

At one point, authorities were heard over a loudspeaker telling someone the building was surrounded and “you need to surrender” and come out with hands up and unarmed.

A shirtless man with blood running down his leg was escorted out of the store in handcuffs by two police officers, a live TV feed showed. An ambulance pulled away from the store, apparently carrying the man. It was not clear whether that man was the suspect, but Yamaguchi said nobody else besides him and those who died was injured.

One person was taken from the shooting scene to Foothills Hospital in Boulder, said Rich Sheehan, spokesman for Boulder Community Health, which operates the hospital. Sheehan said he could not provide additional details but did say that “we have been notified we will not be receiving any additional patients.”

At 4:18 p.m., police reiterated the warning to avoid the area – “this is still a very active scene” – and asked local residents to stay inside and not to “broadcast on social media any tactical information you might see.” The shelter-in-place request was lifted around 6:40 p.m. Mountain Time.

Kim Cordova, President of Local 7, which is both the largest union in Colorado and represents at least 17,000 grocery store workers in the state and Wyoming, referenced news reports that showed “the best of human nature” in the King Soopers workers.

“After gunshots rang out, grocery workers helped customers in the store find safety, directing shoppers to an exit at the back of the store, and assisted one another to escape the danger inside,” she said in a statement.

“No one should have to fear for their lives while they grocery shop or go to work every day,” Cordova, whose union represents 32 workers at the store, added.

Dean Schiller, who had just left the Boulder store, said that he heard gunshots and saw three people lying face down, two in the parking lot and one near the doorway. He said he “couldn’t tell if they were breathing.”