"I thought it was fireworks," he told CNN's Don Lemon.
Dejesus then saw someone kill a police officer. Multiple officers have died following a protest over the killings of two black men by police
The assailant got out of a Chevy Tahoe SUV wearing what Dejesus described as tactical clothing.
"Man had a rifle, AR-15, clear as day. Pretty big magazine. You can see towards the end of the video here that he goes ahead and drops a few mags."
"He had multiple pockets, he had three to four pockets in front of his pants. He definitely looked like he was wearing something in his shirt."
"He got out of there, walked over to the pillar, put a magazine in and started firing."
Attacker seemed prepared:
"It did look planned. He knew where to stand. He had ammo ready."
"They weren't short mags. They were high-capacity magazines."
The shooter is seen standing by a white pillar spraying bullets to the left and right, which Dejesus believes was to draw the police near.
"He was trying to get a commotion going, trying to get cops' attention."
When one officer attempts to engage the shooter one-on-one, he is killed by multiple shots fired at point blank range.
"It looked like an execution honestly. He stood over (the officer) after he was already down and shot him maybe three to four times in the back. It was very disturbing to watch.
"He shot without any fear. He didn't care. (The police officer) wasn't able to wound him at all. He came around the corner and ended up shooting the officer about half a foot away."
A community divided:
"I don't think social inequality should be dealt (with) in this manner. The community can make a point for their movement. I'm all for social equality, but this was not the way to do it. This was a tragedy. This is nothing less than a terrorist attack. That's my personal opinion."