The fire that killed at least 40 people who were trapped inside a Mexican migrant detention center is bringing to light severe conditions of those housed in migrant facilities, activists say.

Video of the Monday incident was posted on Facebook by Equipo De Rescate Cd Juárez, a local organization that assists in emergency events.

It showed people trapped behind padlock doors and trying to kick the lock open frantically in an overcrowded prison as smoke began to build up and guards paced back and forth outside.

“We’ve been working hard to limit this detention, because this is exactly the kind of thing that happens,” said Gretchen Kuhner, the director of the Mexico-based group Women in Migration.

The Mexican government tries to call them other things, but people are detained there, under lock and key, and they cannot leave ... I’ve been here for 25 years in Mexico, and we’ve been working on the sickening cases that we have,” she told NBC News.

While Kuhner has not seen Estancia Provisional de Ciudad Juárez, the facility where the incident occurred, she says that other detention facilities are often converted from immigration offices and rarely have adequate conditions.

One challenge in regulating these facilities is the fact that Mexico denies entry to many outside organizations, says Rachel Schmidtke, a senior advocate for Latin America at Refugees International. She describes the detention centers as “kind of black boxes” shrouded in mystery.Mexico’s Supreme Court ruled March 15 that some of the country’s laws governing immigration detention centers are unconstitutional.Kuhner described the ruling as a victory for many groups that have been working since 2000 to shut these centers down, but added that there’s still work that needs to be done to address overcrowding.Anthony Gonzalez, a Venezuelan migrant who was held last week at the facility where the fire occurred, told Noticias Telemundo that there were hundreds of people housed there — though the capacity was supposed to be just 60.Mexico’s President Andrés Manuel López Obrador claimed the fire was started by the migrants, but some are pushing back against that narrative. Migrants are stripped of their belongings when they enter facilities, even having the laces taken out of their shoes.