Does the FBI sometimes use polygraph tests just to establish truthfulness of a witness, not intended as evidence for a trial?
FBI, CIA and many agencies use them, but they are largely devices to get the subject talking. They know the polygraph measures things easily spoofed, but it makes the subject work harder to lie and keep themselves under control for longer periods. Liars have to work at it. It's not work if the truth is on your side. Truth is effortless - lying (consistently before a seasoned interrogator) is hard.
One prosecutor told me the worst subjects are people who get polygraphed annually for work because they are hardened to the process. This is key - it's the process and formality that presses on the conscience, and it is the conscience they want to plumb, not the oscillating needles.
The CIA claims they ignore the polygraph results altogether - it is their method of questioning (no, not torture, quite the opposite) that teases the information from a human mind - they use tried-and-true genetic responses and brain chemistry science. Factoid - when someone is lying the response travels through a different path in the brain than truthtelling does - and there are ways to put stumblingblocks on that path causing the liar to stumble over their own thoughts and words.