A vintage B-17 bomber experiencing trouble after takeoff crashed into a maintenance shed as it circled back for a landing Wednesday at a busy airport, bursting into flames and killing an unknown number of people, officials and witnesses said.

There were 10 passengers and three crew members aboard the World War II-era bomber as it crashed at Bradley International Airport, north of Hartford, officials said.

Some people on board suffered severe burns, Public Safety Commissioner James Rovella said at a news conference hours after the crash. It is too soon to say how many died, he said.

Five people on board were taken to Hartford Hospital, said hospital spokesman Shawn Mawhiney. He did not have information on their condition.

The plane was five minutes into the flight when pilots reported a problem and said it was not gaining altitude, officials said. It was trying to land when it struck the shed.

One person on the ground was injured in addition to the people on board, officials said.

The plane was civilian-registered and not flown by the military, the FAA said.

Flight records from FlightAware shows the plane crashed about five minutes after it took off. The data shows the plane had traveled about 8 miles (13 kilometers) and reached an altitude of 800 feet (244 meters).

The New England Air Museum is near the airport, New England’s second-busiest. The plane was associated with the Collings Foundation, an educational group that brought its “Wings of Freedom” vintage aircraft display to Bradley International this week, airport officials said.

Brian Hamer, of Norton, Massachusetts, said he was less than a mile away when he saw a B-17, “which you don’t normally see,” fly directly overhead, apparently trying to gain altitude but not succeeding.

One of the engines began to sputter, and smoke came out the back, Hamer said. The plane made a wide turn and headed back toward the airport, he said.

“Then we heard all the rumbling and the thunder, and all the smoke comes up and we kind of figured it wasn’t good,” Hamer said.

Antonio Arreguin said he had parked at a construction site near the airport for breakfast when he heard an explosion. He said he did not see the plane but could feel the heat from the fire, which was about 250 yards away.

“In front of me, I see this big ball of orange fire, and I knew something happened,” he said. “The ball of fire was very big.”

A smaller explosion followed about a minute after the first blast, he said. He saw emergency crews scrambling within seconds.

The fire and smoke were out within about an hour. Flights in and out of Bradley International were suspended after the crash, and the airport was closed.

Only a handful of the roaring, four-engine Boeing B-17s are still airworthy. They were critical in breaking Nazi Germany’s industrial war machine during World War II.

The Collings Foundation said that the same plane in Wednesday’s crash also crashed in August 1987 air show near Pittsburgh, injuring several people. The bomber overshot a runway while attempting to land and plunged down a hillside as spectators waited for the show’s finale.

The foundation said that the plane was hit by a severe crosswind after it touched down and that the right wing lifted into the air, causing it to overshoot the runway. The plane was damaged but later restored.

The B-17 was built in 1945, too late for combat in World War II, according to the foundation.

It served in a rescue squadron and a military air transport service before being subjected to the effects of three nuclear explosions during testing, the foundation said. After a 13-year “cooling off” period, the plane was sold as scrap and eventually was restored. The foundation bought it 1986.

“This is kind of shocking; it’s a loss to lose a B-17,” said Hamer, whose father served in the Air Force. “I mean, there aren’t very many of those left.”