Hundreds of migrants have fled from detention in southern Mexico over the past month, as advocates say migrants are being held in overcrowded facilities with limited access to food and medical care.

More than 900 migrants have fled in five mass escapes from a detention center in the city of Tapachula since late April, according to statements from the National Migration Institute. Virtually everyone who has fled is a native of Cuba, most likely trying to reach the U.S. border by land.

Mexican immigration authorities have detained thousands of migrants — mostly from Honduras and elsewhere in Central America but also from the Caribbean, central Africa and southern Asia — in southern Mexico since January.

The detention centers “are overcrowded, and there isn’t enough medicine and food to go around,” said Ramon Verdugo, who runs the independent Todo Por Ellos shelter in Tapachula. “Some people are only being fed once a day.”

In Mexico’s northern cities bordering the U.S., some 13,000 migrants are waiting in line to formally petition for asylum in the U.S., according to an Associated Press investigation.