When a gunman opened fire at a Boulder, Colorado, grocery store, Sarah Moonshadow, who was there shopping with her son, timed the gunfire to figure out when to flee to safety.

Ten people, including a police officer, were killed in the Monday afternoon shooting at a King Soopers grocery store. A suspect is in custody.

It was supposed to be a quick grocery run for Moonshadow and her son, but they ended up “stalling a bit, looking around,” Moonshadow told “Good Morning America” on Tuesday.

“And at first I was kind of kicking myself, like, if we had left a little bit sooner maybe we could have avoided things,” she said. “But then I realized that we probably could’ve been in a lot bigger trouble had we left any time sooner, because when we had gotten out of the store, there were people lying in the street.”

As gunfire erupted, Moonshadow said her focus was keeping her son close to her — and she worried about what would happen if he stood up too much.

“I just didn’t want him to be too tall and become, like, a moving target,” she said. “Because we couldn’t tell exactly where the shooter was — we knew he was getting closer to us.”

Moonshadow said she tried to focus on the sound and direction the gunfire and counted the seconds in between shots.

“I just looked at my son and I told him — by the fourth shot I started counting — and I told him, ‘We have three seconds. Stay low and don’t look. And just move fast,'” she recalled.

“And he almost hesitated,” she said. “And I just told him, ‘We don’t have another option. We don’t have any other chance to get out of here.'”

After escaping the store, they hid behind a stone, she said.

Moonshadow said she felt she was out of danger once she saw a police officer arrive on scene.

“I tried to flag him down,” she said. But “my son was really scared and he said, ‘No, please don’t go.'”

The officer pulled up to someone lying in the road, she said, and then she heard more gunfire.

“We just started running further back,” she said.

They weaved through several buildings until they found safety at a community member’s porch, she said.

The officer who died in the gunfire has been identified by police as 51-year-old Eric Talley. A member of the Boulder Police Department since 2010, Talley was the first officer to arrive on scene. Talley leaves behind seven children.