The first—and final—seconds of Air India flight 171

The Boeing 787 Dreamliner, packed with 242 people on board, strained to climb in the steamy midday heat. It barely cleared the rooftops of the residential neighborhood just beyond the runway in the crowded Indian metropolis of Ahmedabad.

A mayday call went out from the cockpit. But there was no response to subsequent calls from air-traffic control, according to India’s civil aviation regulator.

Shortly after taking off around 1:40 p.m. local time, Flight 171 slammed into a facility filled with medical students in a fiery crash that left rescue crews searching for survivors in the rubble. Heavy plumes of smoke rose into the sky as emergency responders carried people away from the scene on stretchers.

Indian authorities said the accident likely killed most of the people onboard the flight bound for London and some on the ground. The jet stopped transmitting flight data about 50 seconds after takeoff.

Jayesh Patel was at a friend’s shop some 500 meters to 600 meters away from the crash site on Thursday afternoon when he heard a loud noise.

“I started seeing huge plumes of black smoke in the sky. Soon, people started running helter-skelter on the road," he told Indian broadcaster NDTV. “I saw from afar that the plane had been burnt to a great extent… I have never seen such a scene in my life."

The airline’s chief executive, Campbell Wilson, had just taken off on another Air India 787 Dreamliner bound for Paris, where he was to attend a key industry gathering.

Wilson’s flight was still over India when word of the crash came. About an hour into the flight, it made a U-turn back for New Delhi so the CEO could respond to the accident.

“I would like to express our deep sorrow about this event," Wilson said in a video message posted on X. “The investigations will take time. But anything we can do now, we are doing."

A relative of one of the victims crying upon news of the death of her brother in the crash.

The New Zealand native had taken over as CEO in 2022, after India’s flag carrier had been acquired by Tata Group, one of the country’s biggest conglomerates. The airline, which has close to 200 jets, was privatized after seven decades of government control.

Air India is organizing two relief flights to Ahmedabad from Delhi and Mumbai for passengers’ families. It said a team of caregivers from the airline had arrived at the scene to provide support.

The tragedy is unusual in many ways. Boeing’s 787 Dreamliner hadn’t been involved in a fatal accident since it went into service two decades ago, despite the recent production troubles with its smaller 737 sibling. And while takeoffs are hectic moments for pilots, fatal accidents are far more common upon landing, according to historical data.

Accident investigators will likely focus on why the aircraft’s landing gear was still down and whether its flaps, which are movable flight-control surfaces on the back of the wings, were retracted prematurely, according to U.S. pilots and aviation-safety experts. Flaps, along with slats on the front of the wings, help generate aerodynamic lift during takeoff.

The jet’s captain, Sumeet Sabharwal, had logged more than 8,000 hours of experience, authorities said, while the co-pilot Clive Kundar had more than 1,100 hours in the cockpit.

“There’s some question about whether the flaps are extended or not, which they would need to be for takeoff," said John Cox, an aviation-safety consultant and pilot. The Dreamliner is also equipped with a warning system to alert pilots if the airplane isn’t properly configured for takeoff.

The airplane’s pitch in videos of the brief flight appears to be correct, Cox said, but it increases as the jet gets closer to the ground, which means that the wings are not creating enough lift. With the nose pointing higher, that would create more drag, requiring more thrust and more lift, he said. “If you’re already not producing enough lift," he said, “it’s a very bad place to be."

The plane, which was headed to London’s Gatwick Airport, had reached an altitude of 625 feet when it stopped transmitting location data, according to Flightradar24. The Boeing plane involved was 11 years old.

The fiery crash left authorities searching for survivors in the rubble.

The plane had crisscrossed the globe in recent weeks, with trips to Tokyo, Frankfurt and Melbourne, Australia. The day before it had flown nine hours from Paris to New Delhi, arriving about a half-hour late.

The U.S., U.K. and Boeing said they were sending officials to assist Indian authorities with the investigation into the accident, likely one of the deadliest in years. Debris from the crash was scattered about the neighborhood. Wreckage of the jet’s pointy tail ended up lodged in a building.

Write to Alison Sider at alison.sider@wsj.com and Krishna Pokharel at krishna.pokharel@wsj.com