They should watch more TV.
There's ten years of "Stargate: SG1", a prequel movie, five years of "Stargate: Atlantis", two years of "Stargate: Universe" and at least two "SG-1" movies that the USAlien Congresscritters could be using as "evidence" in this hunt for THE TRUTH.
Should they need more documentary evidence, there are many years worth of "Doctor Who" files that they could catch up with. Those even relate some of the activities of the very important United Nations Intelligence Task-force, U.N.I.T. which has protected the world and the human species since the 1970's.
And they could ask for copies of the super-secret filmed documentaries collected under the project name "U.F.O.", which discusses some of the background of the S.H.A.D.O. planetary policing agency.
There are others, including the "Secret Wars" paper-copy documents, excerpts of which, amended and rheavily redacted, were once published by the Marvel Corporation but those first three should be enough to give the Congresscritters sufficient expert testimonies to begin their highly vital work.
Getting the Goa'uld or the Aesir (the badly named "Asgard") to sit in on a Senate Hearing, even a closed-door, top-secret one could be difficult and sending a subpoena ad testificandum and a subpoena duces tecum to the lifeform called The Beyonder, which, as an aspect of the Cosmic Cube may not even have an email address, would be problematic but some of the human agents are still among us.
And I'm sure Hydra and A.I.M. would be delighted to testify.
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I need to get out more.