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PARIS (Reuters) - Paris’ Gare du Nord - one of the French capital’s main railway stations - reopened on Tuesday after police evacuated it late on Monday to search a high-speed train for suspected militants.

On high alert due to a spate of Islamist attacks and Sunday’s presidential election, police mounted a large-scale operation in response to a tip-off that would-be attackers were on the train arriving from northern France, a source close to the investigation said.

Between 20 and 30 police vans massed around the station with officers wearing balaclavas and carrying assault weapons, a Reuters witness saw. The alert was called off when nothing was found, a source close to the investigation said.

The police were “searching for three individuals suspected of wanting to carry out attacks in France,” the source said.

An alert about the three suspects was circulated to police nationwide last Friday, and the information leaked on the internet, the source said.

A railway employee at Valenciennes, near the Belgian border, sold a ticket to a man who he thought might be one of them and alerted the police.

Police reopened the station, which is the terminus for Eurostar passenger trains from London, early on Tuesday morning.

“End of security checks. Gradual return to normal,” Paris police tweeted. Publicly, authorities gave no other details about the operation.

Over the past two years, France has been hit by a series of Islamist militant attacks in which more than 230 people have been killed.