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AMSTERDAM — A bus with Spanish license plates and containing gas bottles was found near a concert hall where earlier on Wednesday a rock concert was canceled due to a threat, Rotterdam's mayor said.

Ahmed Aboutaleb told a news conference it wasn't clear whether the threat and the bus were connected. The bus's driver was taken into police custody for questioning, he said.

A Spanish counterterrorism official told The Associated Press that the van is not believed to be connected to the attacks that killed 15 people in Spain last week.

"The ring that the police set up around the (concert hall) led to the detention of a bus with gas bottles", Aboutaleb said.

"Whether the bus with gas bottles can be linked to the threat, that cannot now be established."

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He warned against "swift conclusions". A bomb squad was examining the bus.

The concert, which was to feature Californian band "Allah-Las," was canceled around 7 p.m. local time, shortly before doors were to be opened for guests, after a tip from police in Spain, Dutch officials said.

A Rotterdam police spokesperson said Dutch authorities received a tip from Spanish police that there would be a terrorist attack at the Allah-Las concert Wednesday night, and the venue was emptied by police at around 7:30 p.m. local time (1:30 p.m. ET).

Around two hours later, Rotterdam police arrested a man in a white van with a Spanish license plate near the venue, and gas canisters were found, the police spokesperson said. The Dutch army is investigating.

Police investigate a van that contained gas canisters in the vicinity of the concert venue Maassilo, after a concert was canceled because of a terrorist threat, in Rotterdam, the Netherlands, on Aug. 23, 2017. ARIE KIEVIT / EPA

A spokesperson who handles media for the four-piece band from Los Angeles said in an email that "due to a potential terror threat at The Maassilo in Rotterdam, the Allah-Las show was cancelled tonight."

The spokesperson said details about the threat were not clear. "The band is unharmed and are very grateful to the Rotterdam Police and other responsible agencies for detecting the potential threat before anyone was hurt," the band spokesperson said.

In an interview with British newspaper The Guardian last year, Allah-Las band members said they chose the word Allah, Arabic for God, because they were seeking a "holy sounding" name. They said they didn't realize it might cause offense.

Lead singer Miles Michaud told the newspaper that the group received emails from people around the world who said they were offended.

Michaud said the band usually wrote back and explained its intentions were honorable "and mainly they understand."

Police evacuate concert venue Maassilo because of a terrorist threat in Rotterdam, the Netherlands, Aug. 23, 2017. ARIE KIEVIT / EPA

Rotterdam police did not specify the nature of the threat in a statement Wednesday night, but said an investigation was underway.

"Police took this information seriously enough that after discussion with organizers it was decided to cancel the event," the Rotterdam police statement said.

The U.S. Embassy and Consulate in the Netherlands warned Americans to avoid the area.