LONDON (AP) — A fire at an electrical substation knocked out power to Heathrow Airport for most of Friday, forcing Europe’s busiest hub to shut down for roughly 18 hours, causing widespread cancellations and rerouting headaches, and stranding roughly 200,000 passengers.

The blaze started just before midnight on Thursday at a substation about 2 miles (3.2 kilometers) from the airport and took firefighters around seven hours to bring under control. Authorities said they found no evidence that it was suspicious, and the London Fire Brigade said its investigation would focus on the electrical distribution equipment at the substation.

The fire knocked out power to Heathrow and thousands of homes in the area. It affected at least 1,350 flights to and from the airport, according to flight tracking service FlightRadar 24, and the impact was expected to last several days, as passengers try to reschedule their trips and airlines work to reposition their planes and crews.

After power was restored, a British Airways jet touched down just before sunset on Friday after Heathrow lifted its closure order. Further arrivals followed, including a short flight from Manchester in northwest England.

A British Airways flight to Riyadh, Saudi Arabia departed from Heathrow just before 9 p.m. (2100GMT). British Airways says it will run eight long-haul flights on Friday night. The airport plans to operate full schedule on Saturday.

Residents in west London described hearing a large explosion, followed by a fireball and clouds of smoke, when the blaze ripped through the substation.

About 120 flights were in the air when the closure was announced. Some turned around and others were diverted to Gatwick Airport outside London, Charles de Gaulle Airport near Paris or Ireland’s Shannon Airport, tracking services showed.

Lawrence Hayes was most of the way to London from New York when Virgin Atlantic announced the plane was being diverted to Glasgow.

“It was a red-eye flight and I’d already had a full day, so I don’t even know how long I’ve been up for,” Hayes told the BBC as he was getting off the plane in Scotland. “Luckily I managed to get hold of my wife and she’s kindly booked me a train ticket to get back to Euston (station in London), but it’s going to be an incredibly long day.”

Heathrow is one of the world’s busiest airports for international travel, and saw 83.9 million passengers last year.

Friday’s disruption was one of the most serious since the 2010 eruption of Iceland’s Eyjafjallajokull volcano, which spewed clouds of ash into the atmosphere and shut Europe’s airspace for days.

It’s too early to know for sure what sparked the blaze, but the Metropolitan Police force said counterterrorism detectives were leading the investigation because of their ability to find the cause quickly and because of the location of the electrical substation fire and its impact on critical national infrastructure.

The force said that “after initial assessment, we are not treating this incident as suspicious.”

Heathrow said its backup power supply designed for emergencies worked as expected, but it wasn’t enough to run the whole airport. It said it had no choice but to close the airport for most of the day.

The airport’s CEO, Thomas Woldbye, rejected suggestions that Heathrow didn’t have adequate contingency plans, saying the incident was “unprecedented.

“Contingencies of certain sizes we cannot guard ourselves against 100% and this is one of them,” he said.

Nevertheless, the fallout from the fire led to criticism that Britain is ill-prepared to deal with disasters.

“The U.K.’s critical national infrastructure is not sufficiently hardened for anywhere near the level it would need to be at to give us confidence this won’t happen again,” said Alan Mendoza, the executive director of the Henry Jackson Society, a security think tank.

Tom Wells, a spokesperson for Prime Minister Keir Starmer, acknowledged that authorities had questions to answer and said a rigorous investigation was needed to make sure “this scale of disruption does not happen again.”

Heathrow originally said it expected to reopen just before midnight Friday, though with “significant disruption over the coming days.”

Even after flights resume, it will take several days to mobilize planes, cargo carriers, and crews and rebook passengers, said aviation consultant Anita Mendiratta.

“It’s not only about resuming with tomorrow’s flights, it’s the backlog and the implications that have taken place,” she said.

Mark Doherty and his wife were halfway across the Atlantic when the inflight map showed their flight from New York’s John F. Kennedy Airport to Heathrow was turning around.

“I was like, you’re joking,” Doherty said before the pilot told passengers they were heading back to New York.

Doherty called the situation “typical England — got no back-up plan for something happens like this. There’s no contingency plan.”

At Heathrow, a family of five traveling to Dallas showed up in the hopes their flight home — still listed as delayed — would take off.

But when Andrea Sri brought her brother, sister-in-law and their three children to the airport, they were told by police that there would be no flight.

“It was a waste of time. Very confusing,” said Sri, who lives in London. “We tried to get in touch with British Airways, but they don’t open their telephone line until 8 a.m.”

Travelers who were diverted to other cities found themselves trying to book travel onward to London. Qantas airlines sent flights from Singapore and Perth, Australia, to Paris, where it said it would bus people to London, a process likely to also include a train shuttle beneath the English Channel.

Matthew Muirhead was working Thursday night near Heathrow when he stepped outside with a colleague and noticed smoke rising from an electrical substation and heard sirens.

“We saw a bright flash of white, and all the lights in town went out,” he said.

The London Fire Brigade sent 10 engines and around 70 firefighters to control the blaze and about 150 people were evacuated from their homes near the power station.

The blaze knocked out power to 67,000 customers, though most of it was restored by daybreak, the fire brigade said.

Flights normally begin landing and taking off at Heathrow at 6 a.m. due to nighttime flying restrictions. But the skies were silent Friday morning.

“Living near Heathrow is noisy, there are planes every 90 seconds or so, plus the constant hum of traffic, but you get used to it, to the point of no longer noticing,” said James Henderson, who has lived next to the airport for more than 20 years. “Today is different, you can hear the birds singing.”

Associated Press reporters Stefanie Dazio in Berlin; Yirmiyan Arthur in Kohima, India; John Minchillo in New York; Hallie Golden in Seattle; and Christie Hampton, Brian Hannon and David Cohen in Bangkok contributed to this report.