Recognize it? The Fifth Amendment to the Constitution of the United States. Originally, or way back when, government officials forced individuals to take oaths during an "investigation," such as "swear your allegiance to the King and all his doctrine, before we ask you questions, so that we know we are dealing with a loyal subject." On religious grounds, people would refuse the oath, and hence, they were found guilty.
...fast forward to today. The Supreme Court of the United States created, through case law, the "Miranda Warnings" that must be given to individuals who are in custody (ie not free to leave) and who are being questioned about the crime for which they are in custody. There is no written law requiring it - it is case law, it is a legal principle.
If it is determined that a subject is to have been in custody, and questioned about the crime for which he was in custody, and Miranda Warnings were not properly administered...his statements can not be used against him in a criminal case.
*. These are part of the unalienable rights of man - not just US citizens. That being said, the Miranda Warnings only apply in criminal cases in US courts.]
(* - I don't agree with Miranda Warnings, or with the FBI charging this crime in a US Federal Court, only with the idea that the rights of the constitution apply, unless otherwise stated, to all men)
The FBI decided, because lives were at stake, they would interview him right away and gather intelligence to stop any imminent terror plots. Then, they would give him Miranda Warnings, and interview him again. That way, if he stopped talking after being given the warnings - the information could be used to stop the terror attacks, but could not be used against him in a US criminal court.
The FBI went a step further. They used separate agents for the two interviews. Why? The wall. They created a false wall separating the knowledge obtained in the first interview and the "evidence" gathered in the second interview that will be used against the defendant in a US criminal court.
Is there a law that requires this? No. Is there a court ruling requiring this? No.
Then why? Too many lawyers in law enforcement. Law enforcement entities should not be run by lawyers.
The defense attorney for the terror suspect demanded to see any and all emails written by any of the FBI Special Agents who conducted the first interview. Not just in this case, in many cases, defense attorneys demand any and all emails, voicemail, text messages, etc, between Special Agents conducting the investigations.
In the District of New Jersey, a Federal Judge ruled, after finding that FBI Special Agents (in an unrelated matter) had deleted emails and texts between themselves, that the jury would be given instructions to assume the emails and texts included exculpatory evidence, evidence which would favor the defendant's innocence.
This is the world we live in.
"All of the emails are gone," (the Special Agent) said. Pressed by the judge on when she deleted them, she responded, "I have no idea."