Dallas Police Department SWAT team members say they told a sniper that a robot would be carrying a phone — not explosives — down the long hallway where he had holed up during a standoff after his ambush attack that killed five officers on July 7, 2016.

The SWAT members tell The Dallas Morning News about the construction of the bomb that was instead taped to the back of a book and fastened to the robot before being detonated and killing Micah Johnson. The 25-year-old former Army reservist had told them he wanted to kill white police officers.

The use of the robot to deliver explosives was a first for a U.S. police department.

A grand jury this week found no criminal wrongdoing in the officers' use of force.