found a crime, but for the existence of the olc opinion. and he made it very clear, several times, that that was not his position. >> reporter: today, mueller appeared to point the obstruction issue to congress, ramping up the pressure on capitol hill for impeachment. >> the opinion says that the constitution requires a process other than the criminal justice system to formally accuse a sitting president of wrongdoing. >> reporter: mueller also making clear that his personal preference is not to testify, but if forced, he'll stick within the bounds of his report. >> we chose those words carefully and the work speaks for itself. and the report is my testimony. >> reporter: after two years of attacks from president trump -- >> when they talk about obstruction, we fight back! and you know why we fight back? because i knew how illegal this whole thing was. it was a scam. >> reporter: mueller defended his investigation, saying the obstruction probe was paramount.