The attorney for a security guard injured in a 2015 terror attack in Garland, Texas, has filed an appeal in the U.S. Fifth Circuit after the original lawsuit was dismissed in December.

Bruce Joiner is seeking millions in damages from the FBI after he was shot in the leg when two radicalized Islamists opened fire at a "Draw the Prophet Muhammad" event. Joiner has argued that the bureau fomented the attack and had advance knowledge it was likely to happen but failed to stop it.

ISIS later claimed responsibility for the attack, which took place four years ago on Friday, making it the first-ever attack on U.S. soil by the group.

Joiner's first complaint was dismissed in federal court on the grounds of government immunity. Although he is seeking monetary damages, he has maintained his central purpose is to unearth the FBI's level of involvement.

The two attackers, Elton Simpson and Nadir Soofi, drove from Phoenix to Garland, where the "Draw the Prophet Muhammad" event was being held, and opened fire at a security checkpoint.

Security was heightened due to the nature of the event and Simpson and Soofi were shot dead just yards from where they began their attack, but not before shooting Joiner.

Because of other court cases, it became public knowledge that an undercover FBI agent had infiltrated the small cell of radicals in Phoenix, and when the "Draw Muhammad" event was announced, the agent texted Simpson "Tear up Texas." The Department of Justice characterized the text as "innocuous."

Additionally, it later became known that the same undercover agent was in a separate car directly behind the attackers when they opened fire, had been taking pictures of them seconds before the gunfire and then tried to flee the scene once the shooting began.

"It seems like it had to have been one or the other," Joiner's attorney, Trenton Roberts told the Washington Examiner in 2017. "Just a complete botched operation where they [the FBI] don't want the attack to actually take place, or, it's something where they need the attack to take place in order for this guy [the agent] to advance in the world of ISIS."

"And that's really what I think. I think that they thought, ‘he's undercover and in order to advance, he needed to get pictures or video of this attack,' and then that would bolster his street cred within ISIS," Roberts said.

In the prosecution of the ringleader who orchestrated the attack from Ohio, the undercover agent testified in disguise in a cleared courtroom and told the jury that he was surprised when the attack began.

The incident has not escaped the notice of lawmakers.

In 2017, Sen. Chuck Grassley, who at the time was chairman of the Judiciary Committee, pressed FBI Director James Comey for answers about the event, writing that the numerous revelations about the case "[appear] to contradict your statement in 2015 that the FBI did not know that Simpson was travelling [sic] to Garland and did not have any indication that Simpson planned to attack the event."