The brutal attack on the 24-year-old outside The Clifton pub in Sedgley was captured on three different sources of video, said Det Insp Laura Harrison.
One video was taken from a mobile phone within the pub, which quickly circulated on social media, generating widespread anger.
DI Harrison would not normally encourage footage being shared online.
But in this case, she said this helped to garner the support of the Sedgley community, and identify the attackers.
Speaking today, as five men were found guilty of murdering Mr Cox and one of manslaughter, DI Harrison, senior investigating officer in the case, said: "Yes absolutely [there was a lot of public interest].
"A section of the mobile phone footage had quite quickly found its way onto social media.
"Now, that isn't something that I would normally encourage, or condone.
"But actually, in this case, it was really helpful because I think that soon put the community of Sedgley in support of the investigation.
"It was an incident that wasn't going to be condoned by them.
"It was completely unacceptable and senseless [what had happened] and the community of Sedgley supported the police investigation.
"This helped to quickly identify the people we thought were responsible, and then the men have since stood trial."
Mr Cox died after he tried to stop Grant Danbury - who gave evidence at the trial - from being attacked outside the pub, on the night of August 15 last year.
The assailants quickly turned on Mr Cox, where he was repeatedly punched and kicked before he tragically died.
DI Harrison said Mr Cox, a young father and Wolves fan, tried to "do the right thing" but ending up paying the ultimate price.
She said: "Reece was a peacemaker in this. He was a man that was out for a few drinks with a friend and his father.
"He was somebody that saw an incident unfold in front of him and took it upon himself to try and help that person.
"He, in no way, can be held responsible for what happened. He tried to do the right thing and it was just a complete, senseless attack.
"It is a group of individuals that have acted jointly. They have acted together.
"They have turned their attention on somebody that was trying to stop them, in the pursuit of beating up another man, and from that it has cost Reece his life."
Coverage of the case:
Of the three CCTV locations, one was from across the road, another was from within the pub itself, and then there was the mobile phone footage.
A number of witnesses spoke at the trial but DI Harrison said the "main evidence" was the CCTV, which detectives "relied on heavily".
She said: "It gave us a bird's eye view, and with other angles covered, of exactly who did what, and at what point.
"We were able to say, in the whole melee of the fight, exactly who did punch and kick Reece.
"Therefore, it was really clear for the jury to make an assessment of, not only as a group what were their actions, but as individuals as well."
While the CCTV footage provided clear evidence to the jury, it was not an easy watch, particularly for Mr Cox's family.
Wolverhampton Crown Court was shown the footage a number of times as part of the trial.
DI Harrison said: "Reece's family have sat through quite an ordeal of a trial in terms of that CCTV footage.
"It is not nice for them to watch and they repeatedly had to watch it, as have the jury as well.
"Each time you watch it, I think you can't fail to be moved by it.
"I think it is really difficult for anyone without emotional connections to Reece to sit through that, let alone someone in your family, people that loved Reece, it has been really difficult for them.
"But they have conducted themselves exceptionally well and admirably, and they have been there in support of Reece, and have been Reece's voice.
"They have supported the investigation and been very helpful to the police, so it is a difficult time for them."
She added: "They have had to go through an ordeal.
"I hope that the conviction of these people will bring some kind of closure for them."