Is it possible that Amber could've been an undercover space alien on a recon mission from another planet preparing for a violent takeover of earth? Maybe Botham received a mis-delivered package in the mail from this person named "Gort" and it was numerically encrypted and written in an indecipherable manner. Botham might've received the package to Amber the Alien by mistake, and decided to return it to her late one evening when no one was around. Amber the Alien might've been concerned that he would become suspicious and didn't want to risk him possibly going to the feds and ruining years of planning. Perhaps the numerical encryption letter was informing her where she could find her spaceship before returning back to her home planet? Have they done an extensive search to rule that out? Something to think about. It makes perfect sense.
Or better yet, maybe Botham was the space alien? And Amber the Avenger received the numerically encrypted package explaining where Alien Botham could find his spaceship to return home! Is it out of the question that Amber the Avenger was actually working for the feds, trained in planetary exploration and encryption, knowing exactly what the mis-delivered letter meant? Was she was ordered by her supervisor (Captain America) after she provided him the evidence of the letter to exterminate him, but in doing so would require her to be tried for murder and spend a period of time in prison, so as to prevent mass hysteria and panic on a global scale? This actually makes even more sense. The more I think about it, the more convincing it seems of the two possible scenarios.