>> reporter: his attorney says his client is innocent. >> the government has not retained an expert to give an opinion there was mistreatment, misdiagnosis or unnecessary test given to any patient. >> reporter: others suspected something was wrong back in 2010. >> things were being administered and abused incorrectly. >> reporter: the cancer nurse first complained to investigators after observing employ's at dr. fata's clinic. >> to know that a doctor could have potentially brought your death on sooner is sickening. >> a year later, michigan's licensing board could not prove any violation and closed the file. >> reporter: if convicted, the doctor faces up to 20 years in prison. federal prosecutors are still building their case against him as more patients' relatives come forward. linzie janis, abc news, new york. >> up next -- >> a mystery, for half a millennium. it may soon be solved. >> 21st century technology may be the key to unlocking the