KANSAS -- Four people died after a small plane crashed into a building near Wichita's Mid-Continent Airport in southern Kansas on Thursday morning, city Fire Marshal Brad Crisp said.
Four remain unaccounted for, he said. While five people were initially transported to the hospital, three of those patients have reportedly been treated and released.
"Firefighters engaged in a horrific fight for several minutes. We have the fire under control. We are in the process of trying to determine if all the employees and visitors who may have been in the building are accounted for," Fire Chief Ron Blackwell told CNN affiliate KSNW.
About 50 to 60 firefighters responded and were evacuated after the structure was deemed unstable, Wichita police said.
Two of those killed were inside the building when the plane crashed, police said. The pilot was killed, CNN affiliates reported.
One of the injured is in serious condition and another in fair condition, an official with Via Christi Health, a local hospital, told KSNW. The other three, who were in good condition earlier in the day, have been released, the station reported.
FAA and National Transportation Safety Board investigators are on the scene.
"There is a small aircraft that went into the FlightSafety building" near Mid-Continent Airport, said Steve Phillips, a FlightSafety International spokesman.
The plane is a Hawker Beechcraft B200 Super King Air, Phillips said. It was en route to Mena, Arkansas, CNN affiliate KWCH reported.
The FAA said the twin-engine plane lost an engine on takeoff and crashed into a two-story building, KSNW reported. The roof on the building's eastern side has collapsed, the station reported.
No information has yet been released on the identities of those in the plane or in the building. According to one aviation website, the aircraft can carry as many as 13 passengers.
It "crashed into the roof of our training center. I have no information on how many on board," Phillips said. "We have the manager of the training center on site, and he's making sure everyone is OK, but I don't have an answer for that."
The building is across the street from the airport, he said.
There were about 100 people inside the building at the time, Blackwell told KWCH.
Video from KSNW showed thick smoke rising from the building at 10:20 a.m. (11:20 a.m. ET).
Flights were operating normally, though some roads around the airport have been blocked off, the airport said in a tweet.
"The building has sustained serious damage, including the collapse of walls and ceilings," read another airport tweet.
CNN's Jason Hanna and Amanda Watts contributed to this report.