THE evidence of two little boys who had both suffered horrific abuse at the hands of Rachel Trelfa and Nyomi Fee was key to the couple’s conviction.

The two seven-year-olds, one of whom had at first admitted killing Liam Fee after Trelfa and Fee convinced him he had done so, were separated at the earliest opportunity to be interviewed.

Detective Inspector Rory Hamilton, who led the investigation, said specially trained family liaison officers first had to gain the boys’ trust to find out what had happened to the two-year-old who was killed in his home in March 2014.

Once the trust between the children and officers was established, a catalogue of abuse that involved a makeshift cage and series of tortures emerged.

Over five video sessions, each set up by the specialist officers and social workers, the picture of shocking abuse emerged and the boys' corroboration of what had happened to them in the flat in Thornton, Fife, helped convict the killers.

Mr Hamilton said: “The evidence from the boys was the critical part of it.

“Without the evidence of the boys it would have been very, very difficult to get this case to court.”

During their interviews the boys told of being locked in a makeshift cage and tied up with cable ties. They also claimed they were forced to take cold showers as punishment, while one boy claimed Fee stood on his neck until he blacked out.

He also claimed he was shut in a chest of drawers where he felt “dead”.

The boy, who the women tried to blame for Liam’s death, also told of the “panic” on the night the toddler died and claimed Fee forced his hand into the dead two-year-old’s mouth.

She said to him: "Do you want to see what you’ve done to Liam?” and “you’ve killed Liam”.

The boy also told how the two women were worked up over what to do with the cage, shouting: “Where can the cage go?”.

The lengthy interview footage also saw the boys list a catalogue of abuse they had suffered at the hands of Fee and Trelfa over years.

Other evidence in the case involved a neighbour who told of hearing screaming from the flat at around 7.20pm that night, but a call was only made to 999 almost 40 minutes later.

Several calls were also made to Fee’s mother in between, but were deleted before Fee’s mobile telephone was handed over to police.

When the ambulance crew arrived just after 8pm, Liam had post mortem staining and stiffening, suggesting he had not just died when the women called for help.

Police also discovered evidence of online searches by Fee and Trelfa about injuries to a child and whether lesbians could share a prison cell. The pair joked via text message about Liam’s injuries and Fee texted that all children should be drowned at birth.

However, Mr Hamilton said it was the innocent testimony of two seven-year-olds who “could not have concocted such a story” separately that ultimately led to their conviction.