More than 200 alleged Venezuelan gang members have been deported from the US. They were alleged to be part of Tren de Aragua and the MS-13 gang.
A proclamation invoking the war-time Alien Enemies Act of 1798 was signed by US President Donald Trump.
A court ruling blocked the Trump administration from deporting them, but the flights had already departed.
The move was criticised by Venezuela and Amnesty International USA. White House press secretary Karoline Leavitt denied the court ruling had been violated.
El Salvador's Cecot jail can hold up to 40,000 inmates and has been criticised by human rights groups for mistreating inmates.
Supermax prisons are the highest-level security prisons, designed to hold inmates that pose extreme security threats. Cecot prisoners do not receive visits and are never allowed outdoors.
Inmates are housed in soundproof, 7-by-12-foot cells for 23 hours of the day in a permanent lockdown with no contact with other inmates.
Moving to these prisons means it's long-term. It has a powerful administration with officers having ample authority to punish and manage the inmates and offers only very limited activities. | Reuters, AFP, AP