(DALLAS) -- Police in Dallas are trying to piece together what happened in the deadly attack on police officers last week -- downloading and poring over 170 hours of video and trying to get to the bottom of initials "R.B." scrawled by suspected gunman Micah Johnson in two locations during his final hours, Dallas Police Chief David Brown said Monday.
"We don't know the scope of his plans yet," Brown said.
Brown said he and his family received death threats almost immediately after the Thursday night attack, which left five officers dead nine others injured.
"My particular threat was a post from a private Facebook to our Dallas Police Department Facebook," he said. "We've been unable to identify the source of the threat but we're taking it very seriously."
"As a policing family here in Dallas and across the country, there is a heightened sense of awareness around threats we received all over the country," he said.
"I don't want to just single out me. Everyone's experiencing the same type of awareness, increased awareness, because of people who, in my opinion, are not stable, who could do grave damage to us. So we're all on edge," he said.
Suspected gunman Micah Johnson opened fire on cops during a protest against police brutality in Dallas last week. Police said he was looking to kill white people, especially police officers and was upset about two recent, fatal police-involved shootings.
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