PC Neil Doyle, 36, died following an assault in Liverpool city centre in the early hours of 19 December.
Christopher Spendlove, 30, Timmy Donovan, 30 and Andrew Taylor, 29, are on trial at Liverpool Crown Court for the newly-wed officer's murder.
Mr Spendlove denied he had been "happy to wash his hands" of the crime.
He told the jury he went on a family holiday to Abersoch, north Wales, the day after the attack but said he did not tell them what had happened because he was not involved.
The three defendants also deny charges of wounding with intent PC Doyle's colleague PC Robert Marshall, and causing grievous bodily harm to PC Michael Steventon.
Mr Spendlove is alleged to have watched Mr Donovan kicking and stamping on PC Marshall before intervening in the 45-second attack on the three off-duty policemen during their Christmas night out.
He said that, after finding out about PC Doyle's death, he told his co-defendants they needed to "speak to the police and tell them your side of the story".
He added: "In my opinion they had been fighting in town, they had punched a few people, there had been a fatality."
Mr Spendlove said he did not know who was responsible for killing PC Doyle and denies seeing Mr Taylor punch either the murdered policeman or PC Marshall.
Prosecutor Nicholas Johnson QC asked why neither of the co-defendants had admitted punching PC Doyle.
Mr Spendlove replied: "I don't know, they haven't admitted it to me either. They are either scared or don't think it's them. I have asked them."
The jury heard the defendants had spoken in the court back room during the proceedings.
Mr Spendlove said: "I made it clear - 'Someone here has done it'. Both of them said 'It wasn't me'. I have asked the question. They basically put their head down and deny it. I want to know it too."
He added: "I didn't play any role in the incident, that's why I said: 'You two need to figure it out'."
It was suggested that Mr Spendlove took a "gamble" in not saying anything in a police interview after providing his account.
But he disputed the claim, adding that he had taken legal advice to give "No comment" answers following his "full and frank" interviews.
The trial continues.
A man on trial for the murder an off-duty policeman told the court he went on holiday the next day because he "played no part in the incident".