The United Nations chastised Thailand on Monday for a series of murders of land activists which have gone unpunished, highlighting the kingdom's shockingly poor record in solving such killings.
The UN's regional human rights office (OHCHR) said it felt compelled to speak out after an appeals court in the South upheld the acquittal of the sole suspect in the murder of an activist last year.
Chai Bungthonglek was gunned down at his home in Surat Thani in February 2015 by a killer who sped off on a motorbike.
He was the fourth leader of a local group which campaigns against land expropriations by palm oil companies to be murdered since 2010.
No one has been convicted over the killings.
"It is very worrying that after these killings, as well as a number of other attacks, investigations have failed to bring anyone to justice," OHCHR's acting regional representative Laurent Meillan said in a statement.
He added that authorities should undertake "impartial, independent and thorough investigations in all cases of killings and attempted murder".
Thailand has long been an intensely dangerous place in which to take on powerful interest groups.
The military, which seized power two years ago, has vowed to curb corruption and go after "mafia figures" but their stated resolve has seen little change in the lack of policing breakthroughs.
A 2014 report by Global Witness said Thailand was the eighth most dangerous country in the world to be a land rights activist -- and the second most dangerous in Asia after the Philippines.
Rights groups say between 50-60 rights defenders have been murdered in the last 20 years.
There are also at least 81 open cases of enforced disappearance dating back as far the mid-1990s, according to Angkhana Neelapaijit from the Asian Federation Against Involuntary Disappearances.
Angkhana's husband Somchai, a rights lawyer who represented Muslims arrested in the Deep South, disappeared in 2004.
He was last seen being taken into police custody in Bangkok. Five police officers were convicted, but later acquitted, of abduction.
In October authorities officially dropped their investigation, saying no culprits had been found.