Part of a historic six-story apartment building in Davenport, Iowa, collapsed Sunday, leading to the rescue of eight people and injuring an unknown number, authorities said.

Concerns lingered Monday morning that some could still be buried in the rubble after the search for the property continued overnight, city officials said.

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

«A part of the rear building had actually collapsed, detached from the building,» he said.

Onlookers watch as emergency crews work at the scene of a partial building collapse Sunday on Main Street in Davenport, Iowa. Nikos Frazier/AP

Seven people were rescued in the hours after the collapse, while rescuers helped more than a dozen people as they left the building, Carlsten said. Those seven people were treated at the scene, he said, indicating relatively minor injuries.

An eighth person was rescued during an overnight search and was taken to the hospital, Mayor Mike Matson said Monday. His condition was not immediately clear.

Authorities said they were still determining how many people might still be missing. They said the current rescue mission would soon turn into a recovery operation, with K-9 units arriving overnight to help with the search.

The cause of the collapse was still under investigation and city structural experts were expected to examine the building, officials said.

It is not clear if the residents, who were evacuated and helped by the Red Cross, will be able to return. Matson said the building was still structurally in disrepair as of Monday morning.

the building is built of brick on steel and concrete, According to the city’s public library.

Rich Oswald, the city’s director of development and neighborhood services, said property owners had «permitting issues» for the exterior brick work. In addition, he said, the owners had municipal orders to make specific repairs and improvements.

Reports of falling bricks in recent days were related to that work, he said. The condition of the building in downtown Davenport was the subject of numerous complaints from residents, officials acknowledged at the news conference.

«The tenants in this building are quite active,» Oswald said. «They have called the city numerous times with complaints.»

Ownership of the building could not be immediately determined. city ​​documentsas that entities called the 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 housed the Davenport Hotel, the best accommodation in the city at the time. The building is listed in the National Register of Historic Places.

Michelle Acevedo and Tavleen Tarrant contributed.