BERLIN (Reuters) - German police said on Saturday they had arrested a suspect in the case of an alleged extortionist who attempted to force supermarkets into coughing up tens of millions of euros by slipping a potentially lethal poison into baby food. Police in the south-western lakeside city of Konstanz said they would ask a judge to approve an extension of custody for the 55-year-old German man arrested by police special forces late on Friday, after concluding he was a prime suspect.  The suspicions have solidified,  they said in a statement.  In mid-September, a suspect was recorded on security cameras placing quantities of poison in products for sale in some southern German supermarkets. He told the police via email that he done so, and threatened to do it nationwide and elsewhere in Europe unless given money. Police said they had found some baby food spiked with ethylene glycol, a colorless, odorless liquid used in the manufacture of antifreeze, which can be deadly if consumed. Police added that, despite the arrest, the public should remain vigilant for signs of tampering when buying food. 