WA Police are hoping another cold case investigation into the abduction and murder of schoolboy Gerard Ross more than 20 years ago will lead to a breakthrough in the case.

The 11-year-old went missing while on a family holiday in Rockingham, south of Perth, on October 14, 1997.

His body was found dumped in a pine plantation in Baldivis, 20 kilometres away, two weeks later.

A cold case review was initially conducted in 2014 and in 2017 police said they had identified several new persons of interest, but no significant leads resulted.

Gerard Ross was murdered after being abducted while on a family holiday in Rockingham. ( Supplied: WA Police )

On the anniversary of his disappearance last year, Police Minister Michelle Roberts announced a $250,000 reward for any information relating to the case.

On Saturday, police said they were almost a year into a second cold case review of the evidence collected over the last two decades.

Acting Commander Rod Wilde said the scale of the current review made it the second-largest investigation in the state's history, behind the investigation into the Claremont serial killings.

He said every element of the case was being looked at again.

"Every piece of evidence is reviewed forensically. Every lead we have is followed up again and reviewed and investigated," he said.

"We look at all suspects, whether they be deceased, in some cases.

"That's a large list, but we have ways obviously of filtering that list as well."

Claremont murder case provides hope

Despite the significant amount of time that has passed, the recent arrest of Bradley Robert Edwards in relation to the Claremont killings gave police hope the case could be solved.

"I'm very confident that we will get a breakthrough in this case," he said.

"There's someone out there that would know something or have a strong suspicion, could be around their husband, brother, close relative.

The recent arrest in the Claremont killings cold case has given WA Police new hope of finding Ross's killer. ( Source unknown )

"They're the persons that we want to come forward and talk to us."

Ms Roberts said it was important the case was resolved, especially for the Ross family, who have since moved back to Scotland.

"This is a case that has disturbed the West Australian community for some 22 years," she said.

"Have a conscience. Come forward, give some relief to this family.

"People out there must know who is responsible for his abduction and murder."