>>9725829
The Puppet Masters
https://en.wikipedia.org/wiki/The_Puppet_Masters
https://www.goodreads.com/book/show/7171856-the-puppet-masters
So the head of the agency and his two top agents went in and managed to get out with their discovery: an invasion is underway by slug-like aliens who can touch a human and completely control his or her mind. What the humans know, they know. What the slugs want, no matter what, the human will do. And most of Iowa is already under their control.
Sam Cavanaugh was one of the agents who discovered the truth. Unfortunately, that was just before he was taken over by one of the aliens and began working for the invaders, with no will of his own. And he has just learned that a high official in the Treasury Department is now under control of the aliens. Since the Treasury Department includes the Secret Service, which safeguards the President of the United States, control of the entire nation is near at hand.
Evil, slug-like aliens land on Earth. They attach themselves to people and take over their minds. After a while,everyone has to walk around completely naked, so that you can spot the ones who have a slug attached.