At least eight people were injured in an explosion at a residential building in Chicago on Tuesday, authorities said.
Of those injured, three are in serious to critical condition, the Chicago Fire Department said.
The "mass casualty" incident occurred at Central and West End avenues in the city's South Austin neighborhood around 9 a.m. local time.
"This is a confirmed explosion but source of explosion not known," the fire department said.
Six men and two women were taken to hospitals, officials said. Their ages were not immediately clear.
One of the victims was in a building across the street from the explosion, fire officials said.
A building adjacent to the site of the explosion was being evacuated, fire officials said.
First responders have completed primary and secondary searches of the building with the explosion, with a final search currently underway, Deputy Fire Commissioner Marc Ferman told reporters during a midday briefing.
"Right now we feel like we're confident we got everybody out," Ferman said.
The fourth floor of the building had collapsed and was "compromised," he said.
Video from the scene showed debris strewn across the sidewalk, streets and several cars. Windows were blown out of the building, which had extensive damage to its top floor.
Crews for local gas utility Peoples Gas did not smell any gas at the site, a spokesperson for the company said.
"The cause of the incident is unknown, but there is no reason at this point to believe the cause is related to gas or any of our equipment," the spokesperson said in a statement.
The Bureau of Alcohol, Tobacco, Firearms and Explosives and the Chicago Police Bomb Squad were also responding to investigate, the Chicago Fire Department said.
The 35-unit building is owned by West End LLC and managed by Urban Alternatives. It is unclear at this time how many people are displaced, Ferman said.
"This is a devastating event and we are heartbroken for all of our residents," Roman Viere, the building's owner, said in a statement. "Our first concern is the health, well-being and safety of our residents. We are doing everything we can to cooperate with emergency services, and we are ready to do whatever we can to support our residents."
This is a developing story. Please check back for updates.