Six police officers who issued residency permits to migrants in return for bribes of up to several thousand euros have been arrested, authorities said Tuesday.

Four officers were jailed, two were placed under house arrest, and two "intermediaries" were also arrested, the ANSA news agency said, quoting police investigators in Milan.

The policemen worked in the migration office of the Milanese police but, prior to their arrest, had already been moved to other departments as suspicions arose about their conduct.

They are alleged to have accepted bribes ranging from 500 to 5,000 euros (595 to 5,950 dollars) for long-term residency permits, which can eventually lead to Italian citizenship.

Investigations documented more than a hundred cases of corruption from 2013 to 2016, and stemmed from the arrest last year of another allegedly corrupt member of the police's migration office.

The police officer who was apprehended in November 2016 was said to have started selling residence permits to migrants in order to feed a cocaine addiction.