ROTTERDAM (Reuters) - A Dutch judge on Monday extended the detention of four men suspected of plotting a militant attack in the Netherlands, two days after they were arrested in Rotterdam, police said.

The men, aged 20 to 30, had “non-Western” backgrounds, police said, without going into further details.

“They were brought before a judge who decided they should remain in custody,” the force said in a statement.

“The investigation will continue in the coming days, with the emphasis on uncovering the nature and scale of the terrorist threat,” it added.

There was no immediate comment from any lawyer representing the men.

Officers said they seized laptops and other devices during the raids, but no explosives or bomb-making equipment were found.

As part of the same operation, German police arrested a 26-year-old Syrian man in western town of Mainz on Saturday. The arrest was based on an extradition request by Dutch authorities, German police said..

In a separate case, police on Monday said they had detained a 24-year-old suspect in Rotterdam also believed to be involved in “a terrorist crime”.