At least four people were arrested in south-western Germany early Tuesday in a series of raids conducted on suspicion of human smuggling, a spokesperson for the federal police said.

The raids were carried out in Mannheim and in the areas surrounding Karlsruhe and Worms, with a total of 24 properties searched, the spokesperson said.

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A total of 400 police officers were deployed to conduct the raids, with federal police, the public prosecutor's office and central customs involved.

The case pertains to a group of migrants from the Caucasus region that were smuggled into Germany to work illegally and therefore below the statutory minimum wage, including at construction sites, the spokesperson said.

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The investigation was launched after one of the migrants in question was apprehended trying to leave Germany via Franfurt airport, according to police.

Earlier this month, the German government imposed tighter controls at all of the country's land borders in what it called an attempt to tackle irregular migration and protect the public from security threats.

The restrictions are part of a series of measures Germany has taken to toughen its stance on irregular migration in recent years following a surge in arrivals.