"We want this to be public knowledge," Khieu Sopheak said.
The July 12 double slaying of 47-year-old Khim Sambo, a journalist for the opposition Sam Rainsy Party (SRP) newspaper Moneaksekar Khmer, and his teenaged son Khat Sarinpheata brought international condemnation.
Although the government has denied any knowledge of the killings, its questionable human-rights record has made it the target of suspicion from groups including the SRP - the second political party in the country - and New York-based Human Rights Watch.
The FBI also investigated a 1997 grenade attack on an SRP rally in which at least 16 people were killed and SRP leader Sam Rainsy sustained minor injuries. The culprits are still at large.
The ruling Cambodian People's Party won a landslide victory in Sunday's elections, but opposition groups have cited the killings, claiming they could have intimidated voters - a claim the government has angrily denied.