WATCH: Multiple injured in Iowa building collapse

Authorities in Iowa said Sunday that a portion of a historical six-story building located in Davenport has collapsed, leaving an unknown number of people injured with officials concerned that some people may still be under the rubble, reported NBC News early Monday.

The rescue team continued searching for people overnight.

Davenport Fire Chief Mike Carlsten said Sunday: "The collapse just before 5pm may have caused a large natural gas leak and water to leak from each floor."

"A portion of the rear building had actually collapsed, separated from the building," he said.

Carlsten also noted that "seven people were rescued in the hours after the collapse, while more than a dozen people were helped by first responders as they left the building."

Those seven people were all treated on-site as they have minor injuries, he said.

Mayor of Davenport, Iowa Mike Matson said Monday: "An eighth person was rescued during an overnight search and they were taken to hospital."

It remains unclear what was the condition of the rescued person.

Officials said that "the current rescue mission would soon become a recovery operation, with K-9 units arriving overnight to assist in the search."

Authorities are still investigating the cause of the collapse and the city's structural experts are likely to be brought in to "examine the building".

It's not clear whether residents, who were evacuated and being helped by the Red Cross, would be able to return. Matson said the "building was still structurally unsound as of Monday morning".

As per the city's public library, the building is constructed of brick over steel and concrete.

Rich Oswald, the city's director of development and neighbourhood services, said the "property's ownership had permit issues for exterior brickwork."

"In addition, the owners were under city orders to make specific repairs and upgrades."

"Reports of bricks falling in recent days were related to that work," he said.

The condition of the building in downtown Davenport was the subject of numerous resident complaints, officials acknowledged at the news conference.

Oswald said: "The tenants of this building are pretty active. They've called the city numerous times with complaints."

It is yet to be ascertained who owns the building but the city documents show that entities called 324 Main Street Project and the Davenport Project have long-planned improvements to the building while seeking tax breaks.

The building, in the Cork Hill District, was completed in 1907 and was home to the Davenport Hotel, the city's finest accommodations at the time. The building is listed on the National Register of Historic Places.