Windsor Locks — A World War II-era B-17 bomber with 13 people aboard crashed and burned at the Hartford airport after encountering mechanical trouble on takeoff Wednesday, and a state official said at least seven were killed.

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

It had 10 passengers and three crew members aboard, authorities said. At least six people were taken to the hospital, three of them critically injured, authorities said. Additionally, one person on the ground was injured, officials said, and a firefighter involved in the response suffered a minor injury. No children were on the plane.

The state official who gave the death toll was not authorized to discuss the investigation and spoke on condition of anonymity.

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

The Connecticut National Guard issued a news release late Wednesday afternoon saying a member of the Air National Guard was one of the passengers aboard the flight. The member, whose name was not released, "suffered injury" and was taken to Hartford Hospital, the release says.

“We’re closely monitoring the situation as the investigation proceeds,” said Maj. Gen. Francis Evon, the adjutant general of the Connecticut National Guard, which also responded to the scene. “We commend the swift actions of our Guardsmen in supporting the mutual aid response and are grateful for the well-being of our member on the flight. Our hearts are with all of those affected by the tragic accident that took place today.”

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."

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

"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 a Flying Fortress, one of the most celebrated allied planes of World War II — was used to take history buffs and aircraft enthusiasts on short flights, during which they could get up and walk around the loud and windy interior.

It was among vintage aircraft the foundation had brought to the Groton-New London Airport last month.

"Right now my heart really goes out to the families who are waiting," Gov. 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 to investigate the cause of the crash.

The plane was a few minutes into the flight when pilots reported a problem and said it was not gaining altitude, officials said. It lost control upon touching down and struck the shed just before 10 a.m. Flight records from FlightAware shows the plane went down about five minutes after it took off. The data shows it had traveled about 8 miles and reached an altitude of 800 feet.

In recordings of audio transmissions, the pilot told an air traffic controller that he needed to return to the airport and land immediately. Asked why, he said: "Number four engine, we'd like to return and blow it out."

Brian Hamer of Norton, Mass., 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, about 250 yards away.

"I see this big ball of orange fire, and I knew something happened," he said.

Only a few of the roaring Boeing B-17 Flying Fortresses are still airworthy. The planes, 74 feet long with a wingspan of 104 feet, were used in daylight strategic bombing raids against Germany during World War II — extremely risky missions, with high casualty rates, that helped break the Nazis' industrial war machine.

The Collings Foundation said that the same plane in Wednesday's accident also crashed in 1987 at an air show near Pittsburgh, injuring several people. Hit by a severe crosswind as it touched down, the bomber overshot a runway and plunged down a hillside. It later was repaired.

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. It was later sold as scrap and eventually was restored. The foundation bought it in 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."

State and local officials are warning the public about a potential discharge of dangerous firefighting foam into the Farmington River from the deadly crash. A message posted Wednesday on Windsor’s town website advised the public not to come in contact with foam they may encounter on the river or from storm drains, and not to take any fish.

The Department of Energy and Environmental Protection says it’s working with other emergency responders to minimize foam releases into Rainbow Brook, which is immediately adjacent to the airport.

Wednesday’s warning comes after an estimated 50,000 gallons of water and firefighting foam stored at the Windsor Locks airport leaked into the river in June. A state task force has recommended reducing or preventing future releases of firefighting foam.

 Collins reported from Hartford. Chris Ehrmann is a corps member for Report for America, a nonprofit organization that supports local news coverage, in a partnership with The Associated Press for Connecticut.

Day Staff Writer Julia Bergman contributed to this report.