Local police were initially alerted to Abdo by the owners of Guns Galore who reported him as "suspicious."
A clerk at the store, who identified himself to ABC News as Mr. Ebert, said Abdo came to the store Wednesday afternoon looking for gunpowder and "reloading options."
Ebert called the police after he said he felt "concerned with the quantity of his request and his general demeanor."
"There was clearly something wrong with him," Ebert said.
According to the documents, military officials believe the incident "was likely isolated to the Fort Hood area and the suspect in custody, and that arrest of the suspect has mitigated any further threats related to this incident."
Abdo's former lawyer, James M. Branum, declined to comment for this report except to say he hasn't spoken with his client "in a long time." Abdo now faces federal charges in connection with the alleged plot.
ABC News' Matthew Cole, Jason Ryan, Sarah Netter and Kirit Radia contributed to this report.