Heathrow defends its response as questions grow about why a fire shut the airport for so long

The airport reopened after about 18 hours when Heathrow had reconfigured its power supply — something Transport Secretary Heidi Alexander said that "required hundreds of systems to be safely powered down and then safely powered up with extensive testing."

Heathrow said that it ran a full schedule on Saturday and Sunday, with 400,000 passengers passing through on 2,500 weekend flights.

The fire's huge impact raised concern about the resilience of Britain's energy system to accidents, natural disasters or attacks. The government has ordered an investigation into "any wider lessons to be learned on energy resilience for critical national infrastructure."

Counterterrorism police initially led the investigation into the fire, which came as authorities across Europe gird against sabotage backed by Russia. The head of Britain's MI6 spy agency has accused Moscow of mounting a "staggeringly reckless" sabotage campaign against allies of Ukraine, which has been trying to repel Russia's full-scale invasion for more than three years.

Police say they have found "no indication of any foul play," and the London Fire Brigade said that it's leading the investigation, which is focused on the substation's electrical distribution equipment.

Gareth Bacon, the transport spokesman of the opposition Conservative Party, said that "malicious actors ... will undoubtedly have taken note of this weekend's events."

"This episode underlines the urgent need to ensure that our critical infrastructure is safeguarded against both accidental incident and deliberate acts of sabotage by malign actors," he said in the House of Commons.

Meanwhile, the utility company and airport executives are trading blame.

John Pettigrew, chief executive of energy-supply network National Grid, told the Financial Times that "each substation individually can provide enough power to Heathrow" for the airport to stay open.

"Losing a substation is a unique event — but there were two others available," he said. "So that is a level of resilience."

Heathrow said it had worked to reopen "as soon as safely and practically possible."

"Hundreds of critical systems across the airport were required to be safely powered down and then safely and systematically rebooted," the airport said in a statement. "Given Heathrow's size and operational complexity, safely restarting operations after a disruption of this magnitude was a significant challenge."

Heathrow CEO Thomas Woldbye is also facing questions about why he put the airport's chief operating officer, Javier Echave, in charge of decision-making as the fire raged early Friday.

Alexander declined to back Heathrow management's decision-making, saying, "I don't have all the information that they had available when they made the decision."

"Safety should always be paramount, but, as I say, it was not my decision," she told the BBC.