The London-bound aircraft crashed shortly after taking off with 242 people on board on Thursday, leaving a single survivor. A total of 265 people were confirmed dead as of Friday, including medical students at a hostel and other victims in the residential neighborhood where the plane went down, Ahmedabad police said.

An early focus of the investigation into the crash is examining whether the aircraft could have suffered a loss or reduction of thrust, according to people familiar with the probe, who cautioned that many questions remain unanswered.

New points of interest could emerge as investigators comb through wreckage and begin their analysis of the plane’s flight-data and cockpit-voice recorders. The jet’s so-called black boxes were found Friday amid the damaged buildings where the plane went down, according to an official at India’s civil-aviation ministry. The box will be taken to a new lab in New Delhi for analysis, the official added.

Air-accident investigations can last months and often point to a number of factors contributing to a crash, such as possible crew missteps and maintenance errors.

Family members grieving for relatives who died in the Air India crash. Photo: Punit Paranjpe/Agence France-Presse/Getty Images

An ambulance carried the remains of victims from the crash at a hospital in Ahmedabad on Friday. Photo: Saumya Khandelwal for WSJ

India’s civil-aviation regulator on Friday directed Air India to conduct additional maintenance reviews of its fleet of Boeing 787 aircraft with GEnx engines, in what it described as a preventive measure. The airline will be required to perform one-time checks of various systems before departures, including some fuel and engine systems, the cabin-air compressor and hydraulics, along with additional inspections in the coming weeks.

The Air India flight reached an altitude of 625 feet in clear conditions before it stopped transmitting location data, according to Flightradar24, just 50 seconds into the flight. Efforts to contact the cockpit after it issued a mayday call drew no response.

Videos of the flight showed the jet lifting off, then descending toward buildings, sending flames and smoke into the sky.

The surviving passenger, Viswashkumar Ramesh, a British national, had a window seat on an exit row near the front of the plane. He recounted what happened in an interview on Friday from his hospital bed with Indian state-owned television channel DD News.

“I think the side of the plane I was in, it didn’t land on the hostel, it landed below on the ground floor,” he said. “When my door broke, I saw in front of me that there is some space and that I can try to go out. So I tried and got out there.”

Nobody would have been able to get out on the opposite side of the plane, which was against a wall, he said.

“I myself can’t believe how I came out of it alive,” he said. “Because for some time, I was feeling that I, too, am going to die.”

A nurse who treated Ramesh said he had some cuts and abrasions but nothing life-threatening. “He is traumatized but physically he is fine,” she said.

Ahmedabad is one of India’s most populous cities and the largest in Gujarat, the home state of Prime Minister Narendra Modi. Modi went to the crash site on Friday and visited the nearby Ahmedabad Civil Hospital, where he spoke with Ramesh.

Around 100 people injured on the ground were brought to the hospital, including about 50 medical students who were at the dormitory where the plane crashed, a doctor there said Friday.

The flight was carrying 230 passengers, including 169 Indian nationals, 53 Britons, seven Portuguese and one Canadian. The crash was the deadliest civil-aviation disaster since the 2014 crash of Malaysia Airlines Flight 17 over eastern Ukraine that killed 298 people.

The Air India crash was the first fatal incident for Boeing’s 787 Dreamliner, which entered service in 2011. As is the case with other modern aircraft, it is equipped with advanced safety systems that can aid pilots in emergencies.

Videos suggested the aircraft had sufficient thrust to take off initially, leaping off the runway as it should, said Ben Berman, a former senior accident investigator with the National Transportation Safety Board and pilot for a major U.S. airline.

“It climbed really well for the first few seconds,” Berman said. “It leapt off and then immediately sagged and then sank down.”

A possible failure or combination of problems could have resulted in a reduction of “thrust to a very low level,” he said.

Indian authorities are leading the crash investigation. The NTSB will support the probe, with assistance from the Federal Aviation Administration, Boeing and engine maker GE Aerospace.

U.S. officials said they would send resources to help and take action as needed to protect air passengers. “We will not hesitate to mitigate any risks we identify in that process,” acting FAA Administrator Chris Rocheleau said Thursday.

“We have a lot of questions but we have to wait because we have to get authentic information,” Natarajan Chandrasekaran, chairman of Air India owner Tata Sons, said in an interview with CNBC.

Write to Shan Li at shan.li@wsj.com and Andrew Tangel at andrew.tangel@wsj.com

Corrections & Amplifications
Viswashkumar Ramesh was incorrectly referred to as Ramesh Viswashkumar in an earlier version of this article. (Corrected on June 13)