Three people were seriously injured when a small experimental plane crashed into a busy Florida supermarket Monday evening, bursting into flames and sending shoppers and employees fleeing for their lives.

The single engine plane had been in the air for just moments when it lost power and crashed into the roof of a Publix supermarket in DeLand, a city about 20 miles southwest of Daytona Beach, during the store's busy, after-work hours, authorities said.

There were 35 shoppers and 33 employees inside the Publix when the plane hit just after 7 p.m., authorities said.  Customers who dialed 9-1-1 reported smoke and burning amid a chaotic scene.

"It was pure pandemonium in there.  People were just trying to get out," Randy Felix, a customer in the store's deli section at the time of the crash, told ABC News.

"This kid started running towards us and he was just laid on the floor and his legs flailing," said Felix, who ran to help.  "He just kept on screaming over and over."

Five people were injured in the crash, including the plane's pilot and co-pilot.  They remain at Orlando Regional Medical Center in critical condition, according to local affiliate WFTV.

A third man is being treated for burns on his legs and the other two victims have already been released from the hospital, WFTV reported.

Authorities said they do not yet know what caused the crash.  Officials from the Federal Aviation Administration and the National Transportation Safety Board will be on the ground in DeLand today to examine the wreckage.

Eyewitnesses reported seeing the plane in trouble from the moment it took off from the nearby DeLand Municipal Airport.  The Orlando Sentinel reported that initial reports indicated the problem was mechanical.

"One of the engines had some flames pop out of it," an eyewitness told ABC News.  "And we watched it like he was trying to turn around, then it dropped straight out of the sky."

The Publix store is closed today as investigators continue their work.  The plane remains inside the store.

"It's one of those situations; you never know what's going to happen,"  Sgt. Chris Estes of the DeLand Police Department told WFTV. "We're just grateful nobody's been killed and the scene is no worse than it is."