But murder rates in the Northern Triangle countries have been declining in recent years, and economic imperatives are believed to be the most important push factor for the majority of recent arrivals.

More than 90 percent of the most recent migrants are from Guatemala, according to the newly released data. The majority hail from impoverished regions, including the Western highlands, where conflicts over land rights, environmental changes and depressed prices for crops like maize and coffee are undermining the ability of farmers to make a living.

[Read about the treatment of detainees with medical conditions.]

Traveling with children is helping migrants avoid detention

Migrants generally lack understanding of United States immigration law. But they appear to be informed about the basics.

The majority know to request asylum at the border, either at an official port of entry or when they surrender to border agents shortly after sneaking into the country from Mexico.

They know that they are unlikely to remain detained if they travel with a child and that they have a better shot at fending off deportation when they come with a child.

By law, the government cannot keep migrant families in holding facilities at the border for more than 72 hours. It must either transfer them to an immigration detention facility suitable for children or release them.

The government has been letting thousands of detained migrants go free each week because it lacks enough beds to hold them in family detention facilities. Immigration and Customs Enforcement’s three residential family centers — two in southern Texas and one in Pennsylvania — can accommodate 3,326 parents and children.