The fire that killed at least 40 people at an immigration detention center in Mexico happened as Western hemisphere countries face pressure to address the extraordinary number of people fleeing their homes.

Mexico has expanded its network of dozens of detention centers while working closely with the United States to limit movement of asylum-seekers through its territory to the U.S. border, including to Ciudad Juarez, where authorities said migrants set mattresses on fire late Monday in a detention center after learning they would be deported.

Here are some questions and answers about the conditions and policies that led to one of Mexico’s deadliest events at an immigration detention center.

WHY WERE THESE MIGRANTS DETAINED?

Specifics have yet to be released, but Mexico has emerged as the world’s third most popular destination for asylum-seekers, after the United States and Germany. It is still largely a transit country, though, for those on the way to the U.S.

Asylum-seekers must stay in the state where they apply in Mexico, resulting in large numbers being holed up without work in Tapachula, near the country’s southern border with Guatemala.

Tens of thousands are also assembled in border cities, including Ciudad Juarez, often arriving illegally after harrowing journeys or paying someone off. A sprawling network of lawyers, fixers and middlemen has sprung up to provide documents and counsel to migrants who can afford to speed up the system.

More than 2,200 people are believed to be at Ciudad Juarez migrant shelters, and more are living elsewhere in the city after arriving from Guatemala, Venezuela, Nicaragua, Colombia, Ecuador, Peru and El Salvador, according to a report issued last month by the Strauss Center for International Security and Law at the University of Texas at Austin.

Mexico carried out more than 106,000 deportations last year, with about 4 out of every 10 sent to Guatemala or Honduras.

With 28 of the dead identified as Guatemalan citizens, Guatemalans were the largest group among those killed or injured in Monday’s blaze, according to Mexico’s Attorney General’s Office. Others were from Honduras, El Salvador, Venezuela, Colombia and Ecuador.

Guatemalans have been disproportionately affected by a U.S. policy in effect since March 2020 to return people who enter the U.S. illegally to Mexico. The practice suspended their rights to seek asylum on grounds of preventing COVID-19.

Mexico takes back Guatemalans and some other nationalities, while people from other countries are often released in the U.S. to pursue their cases in immigration court. That’s due to the costs and diplomatic challenges of sending them home.

On May 11, the Biden administration plans to end the pandemic-era rule, known as Title 42, and replace it with a sweeping new policy that largely bans asylum for anyone who travels through Mexico without first seeking protection there.

The U.S. Department of Homeland Security received more than 11,000 comments on the new policy before a Monday deadline for public feedback. The U.N. refugee agency said “key elements of the proposal are incompatible with principles of international refugee law.”

The American Federation of Government Employees, the main union representing asylum officers, opposes the change.

The proposal is subject to revisions based on public comment and will almost certainly be challenged in court.

Amid the uncertainty and rapid change, frustration is running high among many migrants about a glitch-plagued app called CBPOne, which was expanded in January to grant some exemptions to the asylum restrictions. The U.S. has been admitting about 740 migrants daily at land crossings through CBPOne.

About 80 migrants are being admitted daily from Ciudad Juarez to El Paso using CBPOne, according to the Strauss Center.