Human rights campaigners believe nearly 2,000 men have disappeared into secret prisons in Yemen.
Human Rights Watch has accused the United Arab Emirates of backing the disappearance and torture of dozens of people in Yemen.
In a separate investigation, the Associated Press news agency says it has evidence of a secret network of prisons in the south of the country where inmates are subjected to routine abuse and torture.
"Unfortunately, there are cases of torture that have come out, but they are anonymous testimonies over fears for the lives of the people who have gotten out, because there are some individuals who were released, and they bore witness to the torture in the prisons and were arrested again," said Huda Al-Sarari, a human rights lawyer.
TRT World's Jon Brain has more.