Investigators combed through the burned-out remains of a power substation near London’s Heathrow Airport on Friday, seeking the cause of a spectacular blaze that shuttered Europe’s busiest travel hub for much of a day and raised broader questions about Britain’s energy infrastructure.

Officials and energy experts said a fault in a transformer with 275,000 volts running through it probably sparked a massive, oil-fueled fire that severed the airport and tens of thousands of nearby homes from the power grid. Systems designed to prevent such a fire apparently failed, and the size of the blaze appeared to keep a second, nearby transformer from restoring electricity.

But the mystery of what caused that fault in the first place remained far from solved by the end of the day on Friday, even as flights resumed at Heathrow.

The Metropolitan Police in London said that counterterrorism specialists had taken charge of the investigation, “given the location of the substation and the impact this incident has had on critical national infrastructure.” At the same time, political leaders and industry experts said it appeared most likely that the fire was an accident.

Both possibilities left residents of Britain and global travelers rattled.

If a malicious adversary can so dramatically disrupt worldwide travel by causing a fire in a neighborhood power station, it raises new concerns about the ability of open societies like Britain’s to guard against such nontraditional attacks.

And if the fire was the result of an undetected weakness in the basic infrastructure of Britain’s power grid, the scope of the chaos that was unleashed could undermine confidence in the nation’s ability to fix crumbling systems at a time when finances are already strained.

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