Dallas police officials are saying that Micah Jonhson, the man accused of fatally shooting five Dallas police officers at a Black Lives Matter protest on Thursday evening, used his own blood to write cryptic messages inside the building where he died.

In an interview on CNN’s “State of the Union” Sunday morning, Dallas Police Chief David Brown said Johnson left the messages throughout the building, reports The Huffington Post.

“We are trying to decipher [the messages], but he wrote the letters ‘RB,’” Brown said. “[W]e’re trying to figure out through looking at things in his home what those initials mean, but we haven’t determined that yet.”

The blood on the walls led investigators to believe that Johnson was injured while he made his way up the stairwell of the building. According to the Huffington Post, officers tried “negotiating with him for several hours before exchanging gunfire and eventually detonating a C4 plastic explosive strapped to a robot bomb-detector.” Johnson was killed in the explosion.

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