Westminster police released a statement saying a man unrelated to the other three victims was also shot, but is expected to survive.
Department spokeswoman Cheri Spottke said a man suspected in the attack is in custody. A motive wasn't immediately clear, she said, AP reported.
Authorities didn't immediately release the suspect's name.
Police said a vehicle seen leaving the area was located about three hours after the shooting, and investigators were interviewing the driver.
Officers who arrived at the scene found four people with gunshot wounds in a business complex parking lot in Westminster, which is about 14 miles (23 kilometers) northwest of Denver.
Investigators were interviewing people who were inside or near the business complex at the time of the shooting.
"A lot of witnesses to talk to and get information," Spottke said. "Obviously, they're upset."
Police surrounded the complex that holds several dental offices, using yellow tape to block access to the area and placing evidence markers. A road next to the business park was temporarily closed to traffic, and police also closed several lanes of another major road nearby.
All roads were reopened by Thursday night. No other information was released.