Aaron’s Zelenski was one of Mueller’s federal prosecutors that lied to the DOJ about sentencing recommendations for Roger Stone. He is one of the corrupt left-wing lawyers associated with Lawfare and Benjamin Wittes.
I can’t imagine what grounds they have for investigating Dr. Zelenko, but does the absolutely corrupt FBI need any?
Trump talked about the medicine that the Doctor used, so that is why. They either assume the worst with Trump, or look for any excuse to make a crime stick.