Investigators said on Thursday that they were following up on a preliminary suspicion of corruption against the mayor over a donation from private property development company Kölbl Kruse to the local Christian Democratic Union (CDU) party.
Kölbl Kruse, based in the western town of Essen, allegedly made four payments of €9,500 from 2008 to 2009 (amounting to €38,000) to the party in exchange for help getting contracts to construct a new state archive at Duisburg's inland port.
The company has been under investigation for bribery for several months, but denies the charges.
The company snapped up the land and property in 2007, beating out competition from the state's own construction and development firm, before then selling its share in the project for €21 million.
Sauerland is not only suspected of knowing the purpose of the donation, but also of tipping off Kölbl Kruse about the contract. Cologne lawyer and CDU politician Rolf Bietmann may also have been involved in the deal. According to a report in news magazine Focus, his legal practice is suspected of manipulating the competition for the contract in Kölbl Kruse's favour.
Investigators have already carried out various raids in connection with the case. Sauerland and Bietmann deny all corruption accusations.
“We have agreed to cooperate fully with the investigators,” Sauerland's lawyer said on Thursday. “I will request access to the records. If there are any reasonable grounds for suspecting, then we will try to clear them up as soon as possible.”