The chairman of the Senate Homeland Security and Governmental Affairs Committee is demanding that FBI Director James Comey explain what appears to be conflicting information about how the bureau monitored and responded to a terrorist attack in Garland, Texas in 2015.

On May 3 of that year, two men, Elton Simpson and Nadir Hamid Soofi, attacked people at an event where people were drawing cartoons of the Prophet Mohammed. It was the first attack on U.S. soil for which the Islamic State claimed responsibility.

Simpson and Soofi had amassed thousands of rounds of ammunition and were wearing body armor. Both were shot and killed soon after they breached security at the perimeter of the parking lot, but not before shooting an unarmed security guard in the leg.

Sen. Ron Johnson, R-Wis., wrote to Comey in April about the attack, after a press report indicating that the FBI was tailing the two men just before the attack. "A recent news report by 60 Minutes revealed that an apparent FBI agent not only traveled to Garland, but was in a vehicle directly behind," the two assailants immediately before the attack began, he wrote.

Johnson pointed out that Comey had previously been quoted saying, "We didn't have reason to believe [Simpson] was going to attack the event, or, in fact, we didn't have reason to believe he had left Phoenix."

"The information contained in the 60 Minutes report about the actions of an apparent FBI agent on the day of the attack appears to be at odds with your statement that the FBI, 'didn't have reason to believe [Simpson] left Phoenix,'" Johnson wrote.

Aside from the letter, Johnson has pressed further saying that in previous communications he had with the FBI and the Department of Justice, he was concerned he wasn't told that an FBI "asset" was tailing the two men.

"Why didn't they intervene? As a member of the Senate oversight committee, I think these agencies should be honest when we actually ask them the questions. But it begs the question, what was the FBI doing in Garland, and why wasn't the agency direct with me when we first started writing letters about this back in 2015?" the senator said, according to Fox News.

Johnson also asked Comey whether a handgun purchased by one of the attackers was connected in any way to "Operation Fast and Furious," an operation by the Bureau of Alcohol, Tobacco and Firearms that allowed the sale of illegal firearms in the Arizona area in hopes of tracking the weapons back to Mexican drug cartel leaders.