Police in Garland, Texas, are refusing to release documents that could show an FBI agent allowed a terrorist attack to go ahead in 2015, in order to avoid blowing his cover.

Police are still saying key documents can't be shared because the investigation is still "pending," even though the police chief said just days after the attack that they did not believe there were other suspects, and the two known assailants were killed on the scene. The 2015 attack was the first ISIS-backed act of terrorism on U.S. soil.

The tight hold on documents by police is making it difficult for a security guard who was shot during the event to learn whether an undercover FBI agent was there at the scene and knew that a terrorism event was being planned. The security guard's lawyer believes the FBI agent was trying to get close to the terrorists and may not have warned authorities of the event in order to keep his cover.

In May of 2015, political activist Pamela Geller was hosting a "Draw Muhammad" contest at the Curtis Culwell center in Garland, a northeast suburb of Dallas. Two radicalized Islamists, Elton Simpson and Nadir Soofi, drove from Phoenix to Garland in a car loaded with numerous firearms and hundreds of rounds of ammunition.

The two men were stopped at a parking checkpoint at the perimeter of the parking lot at the center and opened fire. The security guard, Bruce Joiner, was shot in the knee, and because the event was heavily guarded, Simpson and Soofi only made it a few yards past the checkpoint before they were fatally shot.

Joiner and his attorney now want to learn everything they can about a 60 Minutes report that said an undercover FBI was right behind Simpson and Soofi's car and was taking pictures of the men just before they opened fire.

Court documents reveal that the undercover agent had been in contact with Simpson, and had even texted him weeks earlier, "Tear up Texas."

As the attack began, the undercover FBI agent fled the scene, but was briefly detained by the Garland Police. Part of that episode was caught on camera by the Dallas news station WFAA.

ISIS later claimed credit for the attack, making it the first ever ISIS-sponsored attack on U.S. soil, and at a May 11, 2015 news conference, Garland Police Chief Mitch Bates said, "At this time, we have no evidence that there were any other suspects involved in this attack."

(Credit: WFAA-TV)

The Washington Examiner has requested all documentation from the Garland Police Department that mentioned or made reference to the FBI agent and is currently working through administrative legal issues in hopes of obtaining the documentation, but the "pending" label could be an insurmountable hurdle.

The "case status" box on the investigation report forms on Simpson and Soofi that Garland provided are marked "CLOSED/CLEARED." The same case status is provided on the investigation report into Joiner, the security guard injured at the scene. In a filing with the Texas attorney general, the city attorney's office also stated about the Examiner's request:

"The Chief of Police of the Garland Police Department considers this case to be a pending investigation. The fact that the statue of limitations has not run in this matter is enough, in and of itself, to justify the Chief's determination, as additional information regarding an incident often becomes known long after the original incident and forms the basis for additional law enforcement action."

Representatives from the Garland Police Department and the Garland City Attorney's office have not responded to multiple requests for comment.

Open-government advocates like Kelley Shannon with the Texas Freedom of Information Foundation are all too familiar with the issue of police departments using the "pending" label.

"Unfortunately, some law enforcement agencies use a loophole in the Texas Public Information Act to withhold police records even though a suspect is deceased. The law allows investigative records to be withheld if a case didn't end in a conviction or deferred adjudication, but that exemption is intended for living suspects," Shannon told the Examiner.

"When a suspect is dead, the public should have the ability to see the records and learn what happened," Shannon concluded.

In the legislative session this past spring for the Texas General Assembly, a state representative introduced a bill that would have compelled law enforcement agencies to provide case documentation in instances where the suspect died. However, the bill failed later in the session.

Joiner is now the one leading the push to discover what the FBI knew about the impending attack in the days and months before. Joiner's attorney, Trenton Roberts of Houston, previously told the Washington Examiner he thinks the evidence supports a theory that the FBI knew the attack was imminent, but did little to stop it.

"It seems like it had to have been one or the other," Roberts told the Washington Examiner in April. "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.

Joiner is considering suing the FBI and, due to legal constraints, would have to file his lawsuit by the end of October if he chose to go forward with litigation.