Three people in a flight simulator have died after a real plane crashed into the building in the United States.
The pilot of the twin-engine aircraft also died when the aircraft lost power after takeoff, and smashed into a Kansas Flight Safety centre.
The crash injured at least five others and ignited a fire that sent up towering plumes of black smoke.
Only the pilot was on the plane, but it wasn't immediately clear how many people were inside the building at Wichita Mid-Continent Airport, US, where more than four people were found dead, authorities said.
Four more people remained unaccounted for hours after the crash, but a search was halted after a portion of the building collapsed.
Wichita Fire Marshal Brad Crisp assured onlookers the search would resume as soon as the building was stable.
"We understand that this is a very difficult time, especially for folks who have family members who are working out here and they don't know," Crisp said.
The plane, identified as a twin-engine Beechcraft King Air, crashed into a building that FlightSafety International uses to train pilots to fly Cessna planes, company spokesman Steve Phillips said.
It appeared to strike the top of the building and ignite what Wichita Fire Chief Ronald D. Blackwell described as a "horrific" fire.
Jay Boyle, who works at the airport, said he saw people standing outside and pointing, then spotted the crash site.
"I could see from a distance the cutout in the side of the building where it looked like a wing had gone through and you could actually see the aircraft landing gear through a hole in the building," he said.
The crash did not appear to be significantly disrupting passenger traffic at the airport as planes could be seen taking off from other runways.
Located several kilometres west of downtown Wichita, a longtime aircraft manufacturing hub, Wichita Mid-Continent is used by private aircraft and served by several airlines and their regional affiliates, including American, Southwest, Delta, United and Allegiant.
It saw more than 13,000 departures and about 1.4 million passengers last year, according to the U.S. Department of Transportation.
The crash is the latest in a string of incidents at the airport. In December, an avionics technician was arrested after a months-long undercover sting when he allegedly tried to drive a van filled with inert explosives onto the tarmac in a plot prosecutors say was intended to kill as many people as possible. Then in January, an Oklahoma man rammed his pickup truck through a security gate at the airport. In September, the airport conducted a large-scale disaster exercise featuring the mock crash of a 737 aircraft.
FBI spokeswoman Bridget Patton said it is "too early to rule anything out" about the cause of the crash and confirmed the FBI is assisting in the investigation, but stressed the agency's protocol is to respond to "any and all plane crashes at airports."