Authorities have opened an investigation into one of India’s deadliest aviation disasters after an Air India flight crashed shortly after takeoff, killing 241 of the 242 people on board. The lone survivor remained in critical condition, officials said Friday.

The London-bound aircraft went down in a densely populated residential area of Ahmedabad on Thursday, minutes after departure.

Prime Minister Narendra Modi visited the crash site Friday, calling the incident a “national tragedy.”

Emergency workers scoured the wreckage through the night. Officials said most victims were burned beyond recognition. The cause of the crash remained unknown, and there was no immediate word on whether the flight data and cockpit voice recorders – commonly known as black boxes – had been recovered.

The plane hit a building housing a medical college hostel and burst into flames, killing several college students on the ground.