There were no confirmed fatalities and no known people still trapped the morning following the incident in the city of Davenport, authorities said on Monday.
Rescuers were called to the scene shortly before 5pm on Sunday.
Fire crews rescued seven people and escorted more than a dozen others from the building in their initial response. The building was home to 84 apartments and businesses.
Emergency services used a fire engine ladder to pull seven people from multiple floors of the building, while another dozen people walked out on their own.
With the help of six rescue dogs, first responders pulled an eighth person from the building overnight who was being treated at the hospital, Mayor Mike Matson said at a Monday morning briefing.
“Our focus is rescue right now… This is an active scene. We will continue to work, continue to evaluate, with the whole purpose of trying to find people and trying to get them out,” he said.
Mr Matson credited first responders from city, county and state agencies that immediately entered the building “irrespective of their personal safety” to help victims.
“Because of that, they saved lives. ... They're the heroes of the day,” he added.
The four corners of the building remained standing, but a section of the building a couple of apartments wide collapsed on one side, leaving support beams dangling from upper floors and a pile of rubble below.
The cause of the collapse was not immediately known.
