Six months after a deadly dam collapse in Brazil, new evidence has emerged that suggests the disaster could have been prevented... A prosecutor and lawmaker say German firm Tüv Süd is failing to co-operate. They complain the company's lack of co-operation is affecting their inquiries...

Documents and internal emails seized by investigators show that Tüv Süd employees knew for around a year that there was liquefaction at the dam... In the following months email exchanges show that Tüv Süd's analyses of the dam failed to meet the official safety level required to certify the dam.

Investigators have now established that they eventually "solved" this problem by changing the way in which the dam was certified. They attached conditions of use to the certification: making it, for example, dependent on there being no explosions around the site - unlikely in mining country. This enabled them to sign it off despite its weakness...

Tüv Süd declined the BBC's request for an interview but said they were working intensively towards establishing what happened. Vale says that regular internal and external audits were carried out at the site and that no anomalies were detected that implied an imminent risk of rupture. The German government told the BBC that, while it expected every German company in cases like this to co-operate to the best of their ability, it ultimately had no responsibility in this instance...