In this case, the defendant was convicted of robbing a bank and was questioned by the FBI for two days (29.5 hours) before officials brought him before a magistrate – and he’d signed a written confession. 
The Supreme Court ruled Monday that even where a confession is voluntary, it may not be used in federal court if the defendant was held more than six hourse before confessing.   The 5-4 decision stems from a rule stating that a suspect must appear before a magistrate as soon as possible. 

As a result, for a voluntary confession to be valid, the defendant must appear before a magistrate within six hours.  Writing for the majority, Justice Souter stated that “we have always known what custodial secrecy leads to,” and that without this ruling “federal agents would be free to question suspects for extended periods before bringing them out in the open.”

Source: SCOTUSBlog