On Sunday, for the first time in the history of the United States, the president used social media to announce that he wouldn't be running for re-election. Joe Biden did not make a public appearance, or even release a video, leading some to question whether he was behind the decision to drop out of the race in the first place.

Before long, a theory took shape among users on X claiming that the signature on the letter was not actually written by Biden, per the Daily Mail. Comparisons were made to his recent executive orders and other documents, which showed a number of differences. Others questioned why the presidential seal was missing from the letter, and where Joe Biden, who was last seen by the public on Wednesday, was.

As the Daily Mail reports, users pointed to the fact that in the letter, Biden's signature is underlined, and appears to be missing the "Jr," both of which set it apart from previous signatures. Also notable was the size of the "R."

Among those who spread the theory was pro-Trump hedge fund manager and former Democratic donor Bill Ackman.

"One would think that a letter to the American people from the president announcing his decision to step aside would not be signed using a digital signature and would be accompanied by a photo op and a scheduled conference in the morning that followed," Ackman wrote on X. "Instead we have a digitally signed letter released on the president's X account, an account we know that is run by a staffer, no photo, and no live scheduled address to the American people."

"If this were a hostage situation, that letter would not qualify as proof of life," he added.

Ackman went on to share a post from a user highlighting the aforementioned differences in the signatures. "There's no way," the user wrote. "Biden didn't even sign the letter."

In the letter, Biden said he "will speak to the Nation later this week in more detail about my decision," though no specifics have been announced.