This is the horrifying moment a brave police officer attempted to shoot the Dallas gunman - only for the bullet to bounce off his armored vest and for him to turn around and execute the cop at point-blank range.

The footage, filmed from a balcony on a building opposite the shoot-out shows the Dallas gunman - presumed to be Micah Xavier Johnson - hiding behind a pillar when a cop approaches him from behind.

The stealthy officer then attempts to stop the rampage with a well-placed bullet to the shooter's back - but it bounces off, leaving the killer unharmed and the cop exposed.

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Stand-off: The officer (right) sneaks up on the shooter (left, next to car) then leans out to engage him and to shoot from behind a pillar

Armored: But the killer shrugs off the attack and then sprints towards the cop's position. A witness said the maniac had so many ammunition magazines 'they were falling out of his pockets'

Closing in: The officer turns to his right, thinking that the killer will approach from the inside of the pillars - but instead he comes from the outside, catching the officer facing away from him

Courtesy FOX4

The killer then sprints towards the officer, who ducks behind the pillar.

The officer then apparently turns to his right, expecting the gunman to approach from the interior of the pillars - instead, the killer approaches from the outside, rounding the corner to see the officer facing away from him.

It takes just one second for the gunman to shoot his victim at point-blank range, knocking him to the ground, while masonry explodes around him - presumably fired by other officers.

The attacker then cruelly executes the prone officer before continuing the gunfight.

Three of the five men killed have been named so far - Brent Thompson, Patrick Zamarripa and Michael Krol. It is unclear if any of these three were the one executed in such a manner or if it is another of the two cops as yet unidentified.

Witness Ismael Dejesus said the maniac, who appeared to be carrying an AR-15 assault rifle, initially had no idea the cop was behind him.

'A police officer crept up on the shooter and shot him in the back,' he told CNN. 'But it didn’t faze him - he just turned around and shot him.'

And he said that the brutal execution that followed saw the attacker firing multiple times.

‘The police officer was down and he shot him maybe another three or four times in the back,' he explained.

Shot: The killer then rounds the corner and fires, knocking the officer to the floor with a shot to the back

Dead: The cop is then shot at point-blank range - three or four times, according to the witness - and left for dead, while the shooter continues fighting

Dejesus said this shocking moment shows the attacker must have been well-prepared for the massacre. 'Unless he was a really big guy I can only imagine he was wearing body armor,' he said.

The man - one of at least four killers who opened fire on police during a demonstration about the recent high-profile killings of black men by cops - was also described as being incredibly heavily armed.

'It looked like he was carrying an AR-15,' Dejesus said, referring to a popular model of assault rifle.

And he added that the man had 'lots of 30-round magazines - so many that they were falling out of his pockets.'

Anthony May, a retired ATF explosives enforcement officer, told CNN that the killer knew exactly what he was doing, firing from that spot.

'It appears he was trying to draw police into that area,' he said. 'He is very well trained.'

Acted alone? Gunman Micah Xavier Johnson, a former US Army Reservist who served in Afghanistan, told officers it was only him who attacked police, despite other suspects being arrested