Seven people aboard a vintage World War II plane were killed Wednesday when it crashed shortly after takeoff, erupting into flames at Bradley International Airport, just outside of Hartford, Connecticut, authorities said.

State Police Commissioner James Rovella told reporters at an evening news conference that the families of all but three of the victims had been contacted.

The flight took off at 9:45 a.m. before reporting five minutes later that it was having difficulties, authorities said.

"We observed that the aircraft was not gaining altitude," Connecticut Airport Authority Executive Director Kevin Dillon said.

The plane tried to return to the airport when it crashed at 9:54 a.m., officials said.

There were 13 people on board the Boeing B-17, two pilots, one attendant, and 10 passengers. Another person on the ground was injured when the plane slid off the runway and slammed into a building used to house the airport's deicing equipment, officials said.

Witness Brian Hamer, who lives in Norton, Massachusetts, was less than a mile away from the airport when he spotted the B-17, “which you don’t normally see,” flying low overhead.

Hamer saw smoke coming from the back of the craft and heard one engine sputter.

“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.

Another witness, Antonio Arreguin, was parked at a construction site 250 yards from the crash site when he heard an explosion — and felt the heat from the ensuing fire.

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

Hartford Hospital received six patients from the crash, three were initially listed in critical condition, two in moderate condition and one with just minor injuries, doctors there said.

One of the injured, who survived, is a member of the Connecticut Air National Guard, a representative for the service told NBC New York.

The Federal Aviation Administration said the craft, a Boeing B-17, went down at the end of runway 6 and slid off.

Bradley, the second largest airport in New England, was closed and the FAA put in a ground stop for all arriving flights. One runway of the airport reopened shortly after 1:30 p.m.

Several flights headed for Bradley were diverted to the T.F. Green International Airport outside Providence, Rhode Island, officials said.

The airport — in Windsor Locks, about 15 miles north of Hartford — is hosting a show of vintage World War II craft this week.

Many planes in the "Wings of Freedom" show are owned by the Collings Foundation. Bradley Airport confirmed the B-17 that went down Wednesday is owned by that nonprofit organization from Stow, Massachusetts.

"Our thoughts and prayers are with those who were on that flight and we will be forever grateful to the heroic efforts of the first responders at Bradley," the foundation said in a statement.

"The Collings Foundation flight team is fully cooperating with officials to determine the cause of the crash of the B-17 Flying Fortress and will comment further when details become known."

The B-17 was once dubbed the “Flying Fortress” and played a key role for Allied forces in Europe.

The crashed B-17 had been one of 18 still registered to fly in the United States, according to Connecticut Gov. Ned Lamont.

"The tragedy that happened here may be a source of lessons for others that are still flying these B-17s," Lamont said, adding that investigators have to look "at this plane and the potential causes very carefully."

This is a developing story.