Members of the Irish band U2.

A U2 concert at Stockholm’s Globe Arena was canceled Sunday evening after a gunman was believed to be in the crowd, according to police and media reports.

The arena, the biggest spherical building in the world, was evacuated and thousands of fans were held outside for several hours after the show was called off just before its 8:15 p.m. (1815 GMT) start time.

The promoter Live Nation initially said the show was canceled because of a technical problem, but police later opened an investigation into “a violation of weapons laws.”

According to daily Aftonbladet, security guards had let a man carrying a firearm enter the building, who claimed he was an off-duty policeman. “I forgot to leave it at work,” he was quoted as saying by the paper.

Once alerted, police and organizers decided to evacuate the arena to “conduct a search,” according to a police statement.

The suspect was still not identified on Monday.

Fans, some of whom had traveled from Norway and France, waited until 11:00 p.m. before being told the concert was not going to be held.

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