It is “highly probable” that two prominent critics of the South Sudanese government who went missing in 2017 were executed by security agents, according to a United Nations report released Tuesday.

Prominent lawyer and activist Dong Samuel Luak and opposition member Aggrey Idri were kidnapped from Nairobi in January 2017, just days after a court blocked their deportation to South Sudan.

Both the governments of South Sudan and Kenya have consistently denied knowledge of the men’s whereabouts.

The panel of experts said it had received evidence the two men were kidnapped by South Sudan’s Internal Security Bureau and transported in a commercial plane back to Juba where they were taken to the feared state security headquarters known as “Blue House”.

Citing first-hand testimony, the experts’ report said the men were then transported to another detention centre in Luri, about 20 kilometres (12 miles) west of Juba.

“The Panel has received and reviewed a number of independent reports from multiple, highly credible and well-placed sources. These accounts corroborate each other across a number of key details, leading the Panel to conclude that it is highly probable that Aggrey Idri and Dong Samuel Luak were executed by Internal Security Bureau agents at the Luri facility on 30 January 2017,” read the report.

Dong fled to Kenya in August 2013 after receiving death threats for defending a high-ranking politician accused of treason, and has publicly criticised human rights abuses and corruption by South Sudanese government officials.

Rights groups have criticised Kenya for the unlawful deportation of prominent opposition members to their countries of origin, despite them being recognised as refugees under Kenyan law.

“The UN experts panel’s finding that Dong and Aggrey were most likely murdered days after their abduction while family and friends were stonewalled by Kenyan and South Sudanese authorities shows shocking cruelty,” said Jehanne Henry, associate Africa director at Human Rights Watch.

Seif Magango of Amnesty International said Kenya and South Sudan had “failed miserably” in their duty of care toward Dong and Aggrey.

“Dong was registered as a refugee in Kenya and for him to be kidnapped in Kenya and taken to South Sudan to be executed, is appalling. Both governments must end the blanket denials now, tell the families and the world what exactly happened to the two men and bring those responsible to account.”

South Sudan plunged into civil war in 2013 after President Salva Kiir accused his longtime rival and former vice president Riek Machar of plotting a coup against him.

After multiple failed peace agreements, the two men are meant to be reunited in a unity government within two weeks, however Machar is demanding a delay over several outstanding issues.