Three killers had their murder convictions quashed because a police officer working on the case hid the fact she knew one of the trial jurors.
Det Con Rebecca Bryant, of South Wales Police, was a liaison officer to the family of Lynford Brewster, killed in 2016.
She failed to disclose that her son's girlfriend was sitting on the jury at Cardiff Crown Court.
All three men have been found guilty at a re-trial, which cost about £80,000.
Dwayne Edgar, 31, Jake Whelan, 26, and Robert Lainsbury, 25, were convicted in December 2016 of murder for the "cold-blooded" killing of Lynford Brewster, and sentenced to life in prison.
Ms Bryant, 46, now faces an internal police disciplinary hearing.
She had sat with the family of Mr Brewster throughout the original trial.
However, she told juror Lauren Jones to keep quiet about their relationship, despite offering her a lift to court.
Text messages between them were discovered after one of the killer's solicitors was later made aware of the relationship.
One text from Ms Bryant read: "Don't tell any of them who u r to me tho in case they think I've told u about it although u know I haven't xxx."
Another said: "Remember what I sed though, as long as you don't know any of the witnesses that's fine.... I won't be there hardly and I'm not a witness anyway so that ok u don't need to worry bout that."
All three men had their convictions quashed and a re-trial - estimated to have cost more than £2,500-a-day - was ordered.
The CPS judgement said the police officer "failed to say anything to anybody, although she was aware that the juror was sitting on the murder trial".
"It is clear that the officer initially lied about having any relationship with Ms Jones, although she subsequently admitted contacts with her in interview."
The three defendants were convicted for a second time after a seven-week trial at Bristol Crown Court, and will be sentenced later this month.
Father-of-three Mr Brewster was stabbed to death with a hunting knife during a row with drugs dealer Whelan in June 2016.
South Wales Police said Ms Bryant had not committed any criminal offences but would be investigated by the force's internal disciplinary body.
Assistant Chief Con Jeremy Vaughan said: "Our investigation, which has also been subject of independent review, has not found any evidence that the officer intended to undermine the criminal justice process.
"Following a formal submission to the CPS, the matter will now be dealt with through South Wales Police disciplinary processes."