LONDON (AP) — London's Luton Airport reopened Wednesday after a vehicle fire spread through a newly built parking garage, causing the partial collapse of the structure and major disruption for tens of thousands of passengers.

Dozens of flights were canceled, diverted or delayed at Luton, located about 56 kilometers (35 miles) north of central London, after the blaze erupted late Tuesday.

Four firefighters and an airport employee were treated for the effects of smoke inhalation after the fire.

Officials at the airport, which is a hub for easyJet, Ryanair and other budget airlines running flights to destinations in the United Kingdom and Europe, said flights began to arrive and take off again Wednesday afternoon.

Investigators believe the fire started with a diesel vehicle, according to Chief Fire Officer Andrew Hopkinson of the Bedfordshire Fire and Rescue Service. The recently opened parking garage didn't appear to have sprinklers, he said.

“And then that fire has quickly and rapidly spread," Hopkinson told reporters. More than 100 firefighters were deployed to tackle the blaze, he said.

Authorities said they don’t believe someone started the fire intentionally.

Some passengers had to sleep on the floor in the airport or wait at the nearby railway station after their flights were canceled or delayed.

Nikodem Lesiak, a university student trying to return to Poland, said he spent the night at the station.

“When we got here, we found out Luton is burning and everything is closed, and we were supposed to have our flight at 7:50 today but it was canceled," he said.

Video posted on social media and on the websites of British news outlets Tuesday showed police and fire department vehicles gathered outside the multistory parking structure where the top level was engulfed in flames.

Russell Taylor, 41, an account director from Kinross, Scotland, saw the flames after flying in to Luton from Edinburgh. He said that he first saw a couple of fire engines with a car on fire on an upper level of the garage, which served Terminal 2.

"A few minutes later, most of the upper floor was alight, car alarms were going off with loud explosions from cars going up in flames," he said. "The speed in which the fire took hold was incredible.”