LONDON — Heathrow officials on Monday defended their response to a fire that shut down Europe’s busiest air hub for almost a day, after Britain’s energy system operator suggested the airport had enough electricity from other sources to keep running.

More than 1,300 flights were canceled Friday after a fire knocked out one of the three electrical substations that supply Heathrow with power. More than 200,000 passengers had journeys disrupted, and industry experts say the chaos will cost airlines tens of millions of dollars.

The airport reopened after about 18 hours. Heathrow said it ran a full schedule 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 a probe 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 in its war against Russia’s full-scale invasion.

Police say they have found no sign of foul play. The investigation has been handed back to the London Fire Brigade, which said it is focusing on the substation’s electrical distribution equipment.

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.”