The FBI is under mounting pressure from the Senate to explain the circumstances behind a terrorist attack in Garland, Texas in 2015, although it's still far from clear whether the FBI intends to explain why an FBI agent was at the scene and did nothing.

In open testimony before the committee on Wednesday, FBI Director James Comey told Sen. Ted Cruz, R-Texas, that he would be willing to explain media reports that suggest the FBI may have had advance knowledge of the attack. But he only said he would do that in a classified briefing, and no briefing has been set.

Cruz's request was just the latest attempt to get to the bottom of the attack. Senator Ron Johnson, R-Wis., has been trying to get information from the FBI and the Justice Department for over a year and a half, and has had little to show for his efforts.

Requests for comment from Cruz and Johnson were not returned about when a briefing might occur.

In May 2015, two radicalized Islamic extremists, Elton Simpson and Nadir Soofi, went to a "Draw Muhammed" event in a car loaded with firearms and ammunition. They opened fire at a security checkpoint at the perimeter of the parking lot, and were quickly shot and killed by a traffic officer on the scene before they could reach the building in which the event was being held. Bruce Joiner, a security guard on the scene, was shot in the leg and survived.

A report by 60 Minutes, however, showed a remarkable level of involvement by the FBI on that day. An undercover agent was in the car immediately behind the two attackers, and was taking pictures of the scene just seconds before Simpson and Soofi opened fire. The undercover FBI agent had been in touch with Simpson for weeks prior to the shooting, and even texted him the message "Tear up Texas" weeks before the attack. And just after the pair began their attack, the undercover agent tried to flee the scene.

When Johnson first queried the Justice Department for information that could help him understand what happened that day, he said the response "contained little specificity and ignored several important questions."

After the 60 Minutes report in March, Johnson again requested documentation from the FBI, but his deadline came and went, and the FBI sent him nothing.

The FBI press office told the Washington Examiner it is "in contact" with Johnson's office about the questions he's raised about the FBI's involvement in the incident, but had nothing else to say about it.

Still, that acknowledgment is a big leap forward in the bureau's willingness to make any public admission about the attack, which was the first on American soil for which the Islamic State claimed responsibility.

Toward the end of the Senate Judiciary Committee hearing this week, Sen. Chuck Grassley, R-Iowa, forced Comey to confirm for a second time that he would not only brief Cruz, but also committee staff.

"Assuming they have the clearances for it. I don't think that's a problem at all, I'll do that," Comey replied.

The attorney for the wounded security guard, Trenton Roberts, previously told the Washington Examiner that he was concerned the FBI may have been more invested in moving an undercover agent up the ranks in a U.S.-based cell of the Islamic State than they were interested in stopping an imminent attack.

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

Joiner and Roberts have about six months to decide if they'll sue the FBI, which could lead to more disclosure.