The city's SWAT team, crisis negotiators and law enforcement from nearby communities are assisting, police said. A school near the scene has been locked down.
Some people were evacuated and police sent out a reverse emergency notification to area residents to shelter in place.
Nate Schamel, who lives in an apartment building across the street from Arista Flats, told The Associated Press that he first heard sirens at around 6:45 a.m.
“I heard more and went outside onto my balcony. I saw a Broomfield Pd officer pull up across the street from me, get out with his rifle, cock it and start trotting down the street. I asked what was going on and he told me to go inside,” Schamel said in a text.
He said at 7:30, he called down to an officer who was next to his home and asked what was going on, and the officer told him and his wife to evacuate.
“This was after we had already heard multiple bursts of gunfire (from what sounded like multiple different weapons) and as we were leaving we heard 4-5 more bursts of gunfire,” he wrote.
Broomfield is a city of about 75,000 people roughly 15 miles (24 kilometers) northwest of Denver.