Officials still don't know what caused a fire that led to worldwide air travel chaos.

Live from the UK, this is the Marketplace Morning Report from the BBC World Service.

I'm Leanna Byrne.

So a few weeks back, London's Heathrow Airport, one of the busiest airports in the world, had to close and cancel more than a thousand flights, all because

of a nearby substation fire but knocked out its electricity. Now there's a report out looking

into the incident, which the BBC's Theo Leggett has read for us. Hi Theo. Hello, Leanna. What have

we learned from this interim report? The National Energy System Operator, which runs these things in the UK,

has said the cause of the fire at the substation near Heathrow is still not known. It does add

that the Metropolitan Police in London say that it's found no evidence to suggest the incident

was suspicious in nature. So what we do know so far is that on Thursday the 20th of

March, there was a fire in one of three transformers at a substation near to Heathrow Airport. That

tripped out the other two. That deprived Heathrow Airport of not all of its power, but a substantial

amount of power that affected several terminals and a lot of common systems operated around the airport.

The airport decided to close for safety reasons.

And runway lighting and things like that, essential safety systems were still working and there was backup power.

But the airport closed down anyway because it felt it couldn't guarantee passenger safety.

What this report does tell us is

that power to the airport was restored by about half past six the following morning, but it took

another seven hours to get the airport's internal network up and running properly. And even after that,

Heathrow kept the airport closed to passengers until pretty much the end of the day.

Because again, it wanted to be sure that safety critical systems were running before allowing passengers in.

And the result, as we know, is some 1,300 flights in and out of the airport were either cancelled or delayed.

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