On Tuesday, the Mexican leader said the 15,000 troops his government has deployed to the US border do not have orders to stop migrants from crossing, and vowed to investigate the controversial detention.
International law protects the right of undocumented migrants to cross borders to seek asylum, and Mexico has not typically stopped them from doing so at its northern border.
But Lopez Obrador is facing pressure from US President Donald Trump on the issue, and his government is eager to show results and avoid the punitive tariffs the US leader threatened last month to impose on Mexican goods.
On June 7, the two countries reached an agreement, with Mexico agreeing to reinforce its southern border with 6,000 National Guardsmen. Washington has given Mexico 45 days to take action.
US Border Patrol reported apprehending nearly 133,000 people last month – including many Central American families – as monthly totals have begun topping 100,000 for the first time since 2007. Federal agencies involved in immigration have reported being overwhelmed, depleting their budgets and housing large numbers of detainees in structures meant for handfuls of people.
US House of Representatives on Tuesday night approved a $4.5 billion emergency border aid package to care for thousands of migrant families and unaccompanied children detained after crossing the US-Mexico border.