German police today stepped up their hunt for the driver of a truck that rampaged through a Berlin Christmas market. The sole suspect, a 23-year-old Pakistani asylum seeker was released yesterday for lack of evidence, prompting worries of a killer on the loose.<br/><br/>In a blow to investigators, federal prosecutors announced they had to release the only suspect in custody after finding no forensic evidence to link him to Germany's deadliest attack<br/>in recent years.<br/><br/>The attack comes at a sensitive time for Chancellor Angela Merkel who is running for a fourth term in 2017 but has faced strong criticism over her decision last year to open the<br/>country's borders to refugees.