What clues could point towards a missile strike?

Western countries say they have evidence that an Iranian missile shot down a Ukrainian plane close to Tehran airport on Wednesday.

Iran has denied any involvement.

So what’s the evidence that an Iranian missile was to blame?

What is the video evidence?

Video footage shared on Iranian social media accounts appears to show the moment of impact on Ukrainian International Airlines flight PS752.

The images show a flash followed by a loud bang. The plane then continues to fly, turning back in the direction of Tehran airport, then crashing into the ground in a ball of flame.

BBC Monitoring has indentified the location from where the video was shot to Parand, a residential area west of Tehran.

It’s about 30km from Imam Khomeini International Airport, where the aircraft began its journey.

Rows of housing blocks, a construction site and a storage tank, which all appear in the video, match a Google Earth image of the location, according to BBC Monitoring.

Are there clues in the photos?

There are now many photos of the plane wreckage available on media sites.

These photos, and the subsequent detailed examination of the wreckage itself, will hold vital clues such as burn marks indicating a fire, explosive residue which could point to a bomb on board or possible shrapnel damage from a missile.

An anti-aircraft missile is generally designed to detonate at a distance from the target, sending shrapnel towards it, inflicting maximum damage.

Puncture holes pointing inwards could indicate an explosion or damage from outside the aircraft.

And detailed analysis of the size and shape of any holes could give an indication of the type of missile used.

At this stage, it’s difficult to verify many of the images which are being shared which – it’s claimed – show damage consistent with a missile attack.

It’s claimed some show the nose of a missile from the Russian-made Tor missile system.

The US publication Newsweek quoted senior US officials saying this missile was used to strike the plane.

These photos have not been independently verified yet and we don’t know where or when they were taken, because there are no identifiable landmarks.

Second image (right) of Tor anti-aircraft missile debris, supposedly from near the #PS752 crash site. Still unverified, and still going to be very hard to geolocate it based on what's visible in the image. h/t @Liberalist_30 pic.twitter.com/TQNRp6hopj

End of Twitter post by @EliotHiggins

Questions have been raised about why the nose of the missile in these photos appears to be intact.

However, an expert we’ve spoken to explains that it’s not surprising to see the nose – which contains its guidance system and does not itself impact the target – intact and at a distance from the crash site.

There are reports from inside Iran that the crash site has already been bulldozed and the wreckage cleared away.

This makes the digital evidence gathered so far a vital element in determining what actually happened to the aircraft.

CBS crew just visited the #Ukrainian airlines crash site west of Tehran. Nine am local time. Virtually all pieces of the plane were removed yesterday – say locals. Scavengers now picking site clean. No security. Not cordoned off. No sign of any investigators. pic.twitter.com/hhNJyokhjq

End of Twitter post by @elizapalmer

What about the black boxes?

There are two black boxes.

The flight data recorder captures everything going on with the plane’s systems. This will be crucial in revealing if there were any internal malfunctions.

The voice recorder is designed to contain two hours of audio from the cockpit, and may hold vital clues, as it records what the flight crew were saying.

Iranian officials say it could take up to two months to download data from the black boxes.

Here's the "black box" from the Ukrainian plane that crashed near Tehran earlier this morning. (Iranian state TV) pic.twitter.com/gYTLArU6LO

End of Twitter post by @BBCMonitoring

It takes a long time to investigate because there’s a huge amount of data, says aviation expert Julian Bray. However, he also says that skilled analysts may be able to get an initial idea from the data within a couple of weeks.

There’s also a limit to what the black boxes can reveal if a bomb or missile destroys the aircraft. They will simply stop working, although the voice recorder may pick up a sound wave just beforehand.

Some investigations can last years. A probe into the crash of an Ethiopian plane off Beirut in 2010 took two years to complete, even though the black boxes were sent to France, which has some of the world’s top investigators.

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