Federal police in southwestern Germany raided properties in the cities of Mannheim, Karlsruhe and Worms early on Tuesday, investigating a suspected human trafficking operation, a spokesperson said. 

A total of 400 officers were in action, searching 24 properties, with federal police, the public prosecutor's office and central customs involved. 

Police said at least four people had been arrested in the operation, but that a seven-person group including both men and women was under investigation and more arrest warrants were being sought.

What's the case about? 

Police said the case pertains to a group of migrants from the Caucasus region, smuggled into Germany to work illegally and for less than the statutory minimum wage. 

A police spokesman said the case involved at least a two-figure number of people.

The investigation into the group was launched this January, but investigators found first indications of the case last July, when one of the migrants was apprehended trying to leave Germany via Frankfurt Airport.

He was able to present a passport and visa but no work permit at departure checks. Investigators then found he had been working in Germany during their inspection.

Similar cases were then noted at future checks at Stuttgart Airport and again at Frankfurt's hub. 

Customs then checked several companies, including a cement plant, and found several people working illegally. 

Forged EU ID cards enabled people to work

Police said the group used counterfeit EU personal identity cards to enable the people to take up jobs in Germany. 

They were allegedly paid less than the statutory minimum wage. 

As a result, police said they were investigating suspected cases of human trafficking for financial gain, hiring foreign workers without a permit and forgery offenses. 

The federal customs office in Karlsruhe is also involved because of the suspicion of the misappropriation of social security payments.

Earlier this month, the German government temporarily 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. 

This followed a fatal knife attack in Solingen, rising irregular migration figures in recent years, and gains for the anti-migration Alternative for Germany (AfD) party in state elections, with the party later making gains in Brandenburg as well.

msh/wmr (dpa, Reuters)