A US-based rights group has called on the Thai army chief "to cease interfering" in an investigation into deadly political protests in 2010.
Human Rights Watch (HRW) also urged Gen Prayuth Chan-ocha not to "intimidate critics" with defamation charges.
Thailand's Justice Ministry is investigating the deaths of more than 90 people in violence in Bangkok two years ago.
An army spokesman has refuted HRW's claims, AFP news agency reports.
Anti-government "red-shirt" protesters - many loyal to former Prime Minister Thaksin Shinawatra - occupied key parts of Bangkok between March and May 2010, demanding the government step down.
More than 90 people died over the course of the protests, which ended when armed troops moved in.
The Justice Ministry's Department of Special Investigations has ordered inquests into 19 of the deaths and says it wants to question some of the soldiers, the group said. But no soldier or official has been held accountable for the violence so far.
HRW said Gen Prayuth on 16 August told the investigating committee to stop accusing soldiers of killing demonstrators. He also told investigators not to publicly report findings, it added.
The group also accused Gen Prayuth of ordering an army official to file a criminal defamation case against a lawyer representing the red-shirts and Mr Thaksin.
"Abuses by soldiers took place in full view of the Thai public and the world's media, yet the Thai army chief is now trying to intimidate investigators and critics into silence," HRW Asia director Brad Adams said in a statement.
"The government should prosecute all those responsible for crimes during the 2010 violence, including members of the military, who should not be above the law," he added.
A spokesman from the Thai army said that there was "no way that we can interfere" in the investigation.
"How can the army intervene with ongoing cases? The army has no authority to do that," Col Sunsern Kaewkumnerd told the Agence-France Presse news agency.
Mr Thaksin was toppled in a military coup in September 2006 following months of street demonstrations by "yellow-shirt" protesters, a loose grouping of royalists, ultra-nationalists and the urban middle class.
But in both polls since the coup, Mr Thaksin's allies were elected. A legal ruling forced the first government from power in late 2008, a move that triggered protests that culminated in the violence of 2010.
Yingluck Shinawatra, the current prime minister who was elected last year, is Mr Thaksin's sister.
The sharp polarisation of Thai society into rival red and yellow camps has made it difficult to conduct a credible inquiry - most state institutions are viewed as biased by one camp or the other, says the BBC's South East Asia correspondent, Jonathan Head, in Bangkok.
However, red-shirt leaders now complain that the new government has shown little sympathy for their grievances, but has instead prioritised rebuilding relations with the military, our correspondent adds.