The military-backed government in Thailand is probing the anti-drugs campaign launched by the ousted Prime Minister, Thaksin Shinawatra.
Mr Thaksin's war on drugs drew criticism from human rights groups
The campaign, between 2003 and 2004, led to the deaths of more than 2,500 people.
The UN has previously expressed concern about the campaign, amid claims of extra-judicial killings by police.
Mr Thaksin's government said the deaths were largely the result of violence among drug gangs.
According to the Thai News Agency (TNA) website, Jarun Pukdithanakul, Permanent Secretary for Justice, told reporters that officials were now collecting evidence after complaints were lodged.
He called on the relatives of those who died to come forward with evidence.
Announcing the offensive against the drugs trade, Mr Shinawatra, said brutal measures were needed, because drug dealers and traffickers would destroy the nation if left unchecked.
But the methods of the campaign sparked criticism from human rights groups in Thailand and abroad.
Thai police and security forces were accused of murdering drugs suspects, but the government maintained that most of those who died were murdered because of gang rivalries, or were killed in self-defence.