Doesn’t lend a lot of confidence in your investigative skills if you needed the perps voluntary statement in order to convict him.
Have him take the 5th a bunch of times in front of a jury...then present your evidence....
I see you are skilled in court room activities. Tell me this, how do you get him to the stand to offer testimony when his counsel says he is not testifying on his own behalf assuming you know you cannot subpeona him to testify for the prosecution.
BTW, I would rank my skills, what they were, against the best, I was so honored.