Christoph Metzelder, the former Germany and Real Madrid defender, is being investigated over allegations he distributed child pornography.
The 38-year-old is accused of sending images with child pornographic content via WhatsApp to a recipient in Hamburg.
He is said to have co-operated with the authorities with Hamburg Police carrying out two searches in Dusseldorf earlier this week.
Metzelder, who retired in 2014, is best known for his seven-year spell with Borussia Dortmund.
He began his career at Prussia Munster but moved to Dortmund after just one season, going on to be a big success at the Westfalenstadion.
In 2007 Metzelder signed for Real Madrid, and three years later he returned to Germany with Schalke.
Metzelder was also a member of the Germany international team for seven years, winning 47 caps and playing at World Cups in 2002 and 2006, as well as Euro 2008.
