German police say their warnings were not heeded about the dangers of holding the Love Parade music festival in the city of Duisburg.
The death toll from a stampede at the event has risen to 20. The revellers died when panic broke out at the techno music gathering on Saturday.
Authorities say the number of injured now exceeds 500, including 43 seriously hurt, one in critical condition, Reuters reports.
The head of the national police union says he warned event organisers a year ago that Duisburg was not a suitable location for the Love Parade, because it was too small and cramped.
Media reports say the festival attracted about 1.4 million people.
German Chancellor Angela Merkel expects a "scrupulously thorough" investigation into the tragedy, her spokesman Ulrich Wilhelm said.