CONNECTICUT, USA (AP) — A World War II-era plane with 13 people aboard crashed and burned at the Hartford airport after encountering mechanical trouble on take-off Wednesday, killing seven of them.

The four-engine, propeller-driven B-17 bomber struggled to get into the air and slammed into a maintenance building at Bradley International Airport as the pilots circled back for a landing, officials and witnesses said.

It had 10 passengers and three crew members, authorities said.

Connecticut Public Safety Commissioner James Rovella said hours after the crash that some of those on board were burned, and “The victims are very difficult to identify.”

Some of the survivors of the crash were critically injured, authorities said. One person on the ground was also hurt in the crash and a firefighter involved in the response suffered a minor injury. No children were on the plane.

The death toll of seven could rise, Rovella said. He said some lives were likely saved by the efforts of people including a person who raced to help the victims and people on the plane who helped others to escape the fire by opening a hatch, Rovella said.

“You’re going to hear about some heroic efforts from some of the individuals that were in and around that plane,” he said.

The retired, civilian-registered plane was associated with the Collings Foundation, an educational group that brought its Wings of Freedom vintage aircraft display to the airport this week, officials said.

The vintage bomber – also known as Flying Fortress, one of the most celebrated Allied planes of World War II – was used for taking history buffs and aircraft enthusiasts on short flights, during which they could get up and walk around the loud and windy interior.

“Right now my heart really goes out to the families who are waiting,” Governor Ned Lamont said. “And we are going to give them the best information we can as soon as we can in an honest way.”

The National Transportation Safety Board sent a team of 10 to investigate the cause of the crash.

The plane was a few minutes into the flight when the pilots reported a problem and said it was not gaining altitude, officials said. It lost control upon touching down and struck a shed at a de-icing facility just before 10:00 am.

The airport, New England’s second-busiest, was closed afterwards but reopened a single runway about 3½ hours later.