A small plane crashed into a building near Wichita's Mid-Continent Airport in southern Kansas on Thursday morning, killing at least two people inside, leaving four seriously injured and at least five unaccounted for, authorities said. A federal official has confirmed to ABC News that the incident is not related to terrorism.

"There is a small aircraft that went into the FlightSafety building," near Mid-Continent Airport around 10 a.m. Thursday, according to Steve Phillips, a FlightSafety International spokesman.

At the time of the crash, some 100 people were reportedly inside the FlightSafety International building, which provides flight training and houses several simulators, fire officials said, adding that they are attempting to determine the exact number of people possibly in danger of the situation.

Four victims have been rushed to a local hospital to treat "serious" injuries, Wichita Fire Department Chief Ron Blackwell confirmed. One person suffering from critical injuries was transported to Via Christi Health, an official told CNN affiliate.

"The injuries are serious injuries," Blackwell said. "We're grateful a large number of people appear to have gotten out of the building."

Shortly after takeoff, the plane's pilot reported losing engine power and crashed into a two-story building across the street from the airport when trying to return to the runway, the Federal Aviation Administration said in a statement.

A Hawker Beechcraft King Air with a twin-engine, the plane was headed to Mena, Ariz., when it "crashed into the roof of our training center. I have no information on how many were on board," Phillips said. "We have the manager of the training center on site, and he's making sure everyone is OK," Phillips said.

"We don't know what may have caused the incident," Blackwell added, noting that emergency crews faced a "horrific firefight for several minutes."

"They finally got the fire out. You can't tell it was a plane, it looks like a go-kart frame," Ryan Weatherby told the Daily News around 10:45 local time after witnessing the fire.

FAA investigators were on the scene, and the National Transportation Safety Board had also been notified of the incident, CNN affiliate KSNW reported.

Weatherby, who owns an airplane detailing service, said he was working nearby when he noticed smoke emanating from the building, New York Daily News reported.

"I don't see how they would have gotten out of that on top of the building," Weatherby said. "It was definitely a scary situation."

Meanwhile, flights are operating as normal, though a roadway is closed as emergency vehicles respond, the airport wrote on Twitter.