Around 3,000 migrants set out Sunday on what they call a mass protest procession through southern Mexico to demand the end of detention centers like the one that caught fire last month, killing 40 migrants.

The migrants set out early Sunday from the city of Tapachula, near the Guatemalan border. They say their aim is to reach Mexico City to demand changes in the way migrants are treated.

“Now we have decided to move forward in this journey because of the 40 dead who died up there, who set fire to them, they are murderers, they don’t want the people of our countries, and we want justice to be done,” said Juan Carlos Hernandez, a Honduran migrant.

But in the past many participants in such processions have continued on to the U.S. border, which is almost always their goal. The migrants are mainly from Central America, Cuba, Venezuela, Ecuador and Colombia.

Mexican authorities have used paperwork restrictions and highway checkpoints to bottle up tens of thousands of frustrated migrants in Tapachula, making it hard for them to travel to the U.S. border.

Organizer Irineo Mújica said the migrants are demanding the dissolution of the country’s immigration agency, whose officials have been blamed – and some charged with homicide – in the March 27 fire.

Mújica called the immigration detention centers “jails.”

Mexican prosecutors have said they will press charges against the immigration agency’s top national official, Francisco Garduño, who is scheduled to make a court appearance Tuesday.

Federal prosecutors have said Garduño was remiss in not preventing the disaster in Ciudad Juarez despite earlier indications of problems at his agency’s detention centers. Prosecutors said government audits had found “a pattern of irresponsibility and repeated omissions” in the immigration institute.

The fire in Ciudad Juarez, across the border from El Paso, Texas, began after a migrant allegedly set fire to foam mattresses to protest a supposed transfer. The fire quickly filled the facility with smoke. No one let the migrants out.

AP Video by Raul Mendoza.