…or MIST for short, is a team within the FBI that investigates and suppresses Mitochondria-related…stuff in the world of Parasite Eve. It was founded after the first game and Aya is a member of the LA branch in the second game.
They then disappear.

The wiki claims that two of their members plan to make a bunch of Mitochondria-related stuff public at the end of the second game, but this also just kinda vanishes from the story.

Aya herself was probably forcefully recruited into MIST.

Despite being FBI, they operate more like a bounty hunter organization, with individual agents being paid per creature killed. In company scrip of course, not actual money. Because of course.