A new court filing says the FBI installed a camera outside the Phoenix apartment of two Islamic State followers in 2015 before they drove to suburban Dallas to carry out an attack at a Prophet Muhammad cartoon contest.

But lawyers for the only person convicted in the attack say authorities didn't reveal the existence of the camera until three years after his trial.

Abdul Malik Abdul Kareem was convicted of providing guns to the two attackers, but was in Arizona when his friends, Elton Simpson and Nadir Soofi, launched the attack in Garland, Texas. They were killed in a shootout with police.

Kareem's attorneys say the camera was installed outside the apartment on the day they left for Texas.

The FBI and the U.S. Attorney's office declined to comment.