By Aditya Kalra

NEW DELHI (Reuters) -Air India's deadly plane crash that killed at least 270 people has triggered a wave of speculation about what led to the world's worst aviation disaster in a decade, but authorities are slowly narrowing down areas of investigation.

The Boeing 787 Dreamliner is one of the most advanced jets in service and experts say it has a generally strong safety record, with no previous fatal accidents.

Here's what is known about the air crash so far:

VISUAL EVIDENCE

One of the strongest pieces of evidence that investigators are reviewing is a 59-second CCTV video clip that clearly shows the takeoff and the crash of the plane from Sardar Vallabhbhai Patel International Airport in Ahmedabad city, western Gujarat state.

The CCTV camera was located on the far left of the runway near a wall with barbed wire.

The video shows that the Tata Group-owned Air India plane takes off, gains some altitude, flies flat for a couple of seconds, and then starts descending with its tail down.

The descent of the plane starts roughly 17 seconds after takeoff. There is no fire visible around the engine or elsewhere when the plane appears to start going down, and has already crossed the airport boundary wall.

The landing gear visibly remains open throughout the clip.

It took roughly 33 seconds from wheels-up for the plane to crash, erupting into a huge fireball.

WHERE DID THE PLANE FALL

The plane crashed into the B.J. Medical College hostel.

Images of the dining area shortly after the incident showed wheels and other parts of the aircraft embedded in the walls, while debris and belongings of the students, including clothes and books, lay scattered on the floor.

Steel tumblers and plates still containing food lay on the few tables that were left intact.

A strong stench of jet fuel hung in the air at the site on Friday, as authorities used cranes to remove charred trees and debris.

POSSIBLE CAUSES

Air India officials and scores of Indian government investigators have been at the crash site since Thursday. No initial findings have been disclosed so far, but investigators are considering at least three key possible issues, according to a source with direct knowledge.

Investigators are assessing if there are any issues related to engine thrust, which is the force produced by the engine to push it forward through the air. Investigators are also looking at any possible operational issues with flaps. Anti-terror squads have also been part of the investigation teams.

Officials are also assessing why was the landing gear was open for so long after takeoff. A possible bird strike is not a focus of the probe.

The source added that officials are also looking at any possible fault of Air India, including any possible maintenance issues.

WHERE'S THE BLACK BOX?

India's aviation ministry has said investigators and rescue workers recovered the digital flight data recorder - one of the two black boxes on the plane - from the rooftop of the building on which the jet crashed.

There was no information on the cockpit voice recorder, the other black box, which is also crucial to the probe.

India's aviation regulator has conducted additional maintenance actions on Air India's Boeing 787-8/9 aircraft equipped with GEnx engines, including "one-time check" of the take-off parameters before the departure of every flight from midnight of June 15.

The airline has also been asked to conduct electronic engine control tests and engine fuel related checks.