At least four people died inside a Kansas airport building where a small plane crashed at Wichita's Mid-Continent Airport, authorities said Thursday.

At least five others were injured and Wichita Fire Chief Ronald Blackwell said at least four people were unaccounted for hours after the Thursday morning incident, but a search was halted after a portion of the building collapsed. Only the pilot was in the plane and two people confirmed dead were found inside the building, Blackwell said.

Wichita Fire Marshal Brad Crisp said the search would resume as soon as the building was deemed 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. "This is also a very difficult time for first responders."

One person was listed in critical condition at Via Christi Hospital St. Francis and three others were in fair condition, hospital spokeswoman Roz Hutchinson said.

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 Thursday's 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."

Federal Aviation Administration spokesman Tony Molinaro said a twin-engine Beechcraft King Air 200 reported losing engine power just after takeoff. The plane crashed into the FlightSafety Building -- which is used to train pilots to fly Cessna planes -- on airport property while attempting to return to the runway, he and airport officials said.

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The building sustained serious damage, including the collapse of walls and a portion of the ceiling, airport officials said in a statement. Roughly 100 people, including employees and visitors, were inside the building at the time of the crash.

"It was horrific. There was heavy smoke on the horizon as you approached the airport for miles," Blackwell said. "It was a very challenging fire as you might imagine."

Ryan Weatherby said he and several other employees at Yingling Aviation, across the street from the airport, ran outside when they heard the crash and saw rolling black smoke and wreckage.

"It wasn't that loud," he said. "It was more like a screech or something."

Jay Boyle, who works at the airport as a senior field technical adviser, 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.

Wichita Mid-Continent Airport is used by private aircraft and served by several airlines and their regional affiliates, including American, Southwest, Delta, United and Allegiant. According to U.S. Department of Transportation figures, there were more than 13,000 departures and about 1.4 million passengers in 2013. It is several miles west of downtown Wichita, where aircraft manufacturing has long been an important part of the economy.

The crash did not appear to immediately affect passengers coming in and out of the airport, and least one plane was seen departing shortly after the crash.

FoxNews.com's Joshua Rhett Miller and The Associated Press contributed to this report.