A huge fire which broke out at Nottingham railway station is being treated as arson, police say.
Around 60 firefighters extinguished the blaze, which started in a block of toilets and spread to the main concourse and roof.
"Enquiries have been ongoing today and we now have reason to believe the fire may have been started deliberately," superintendent Sandra England said.
"Officers are working to identify anyone who may have been involved in the incident, and we are appealing for information from members of the public."
No one is believed to have been hurt in the fire, which caused major transport disruption and closed the station.
In a video posted on Twitter, incident commander Bryn Coleman said crews had been "offensively firefighting" using breathing apparatus, with firefighters in "some real severe conditions in terms of heat and smoke".
He told Sky News that officers were now investigating and appealing for witnesses.
"Obviously the CCTV is a crucial part of the investigation. There's a lot of CCTV in the station that we need to seize and locate," he said.
Fire officer John Mills told the Nottingham Post it was the worst blaze firefighters in the city had seen for 20 years.
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"This is about complexity rather than the size of the fire," he said. "People driving through the main roads in the city will barely see any smoke.
"That is because the fire is burning continuously through voids, which puts it out of sight and makes it incredibly difficult for us to locate."