The group, Cristosal, has investigated prison deaths and torture under President Nayib Bukele. Its employees were threatened and surveilled, its director said.
The well-known Mexican boxer was detained by U.S. immigration agents in California on Wednesday, days after fighting a high-profile contest against the former YouTuber Jake Paul.
Tens of thousands of people were jailed as part of President Nayib Bukele’s crackdown on gangs. Some police officers now admit they arrested people on flimsy or nonexistent evidence to meet quotas.
Relatives watched as recently arrested men were transferred to prison in April 2022, early in an anti-gang campaign that has imprisoned about 80,000 people.