There is no current law that mandates the separation of families at the U.S. border.

But since the Trump administration rolled out its so-called “zero-tolerance” policy last month, separation of parents from their children has become a more routine part of the process.

Here’s why. Critical to this change is the administration’s decision to call for the criminal prosecution of all adults who illegally cross into the United States. According to existing U.S. immigration enforcement procedure, when a parent is referred for prosecution, the adult is sent to a federal jail and any children are placed in the care of an adult sponsor or, if no close family member or other adult is available to sponsor the child, to a shelter.

Former President Barack Obama also faced criticism for his administration’s use of family detention centers, especially amid the 2014 surge of Central American migrants arriving at the border, often fleeing violence in their home countries.

The federal government has released some preliminary data on family separation in the weeks following the Trump administration’s new policy but the full impact on families at the border is not immediately clear. (More on this data below.)

What follows is a timeline that details how the Trump administration came to embrace this policy.

March 2017

Amid a flurry of executive actions that dealt with the border wall, so-called “sanctuary cities,” and a travel ban that targeted Muslim-majority countries, reports start to emerge that the Department of Homeland Security was discussing the possibility of implementing a policy of separating children from their parents at the border. Sources tell Reuters in early March 2017 that the Trump administration’s goal is to deter mothers from bringing their children on the oftentimes perilous journey to the U.S. border. When CNN asks then-DHS Secretary John Kelly about the potential policy days later, he said, “Yes I’m considering, in order to deter more movement along this terribly dangerous network. I am considering exactly that.”

Weeks later, after facing criticism for the comments, the DHS head appears to walk back the statement.

August 2017

The idea of a family-separation proposal never quite went away, a source familiar with the discussions among Department of Homeland Security officials tells The New Yorker. A proposal for family separation keeps coming up in discussions on broader measures targeting illegal border crossings. “It would rear its head again,” the person said.

October 2017

The Trump administration starts to increase prosecutions of illegal entry in October, according to a data analysis by The New York Times. The newspaper also finds that from October 2017 to April 2018, 700 families were separated at the border, including at least 100 children under age 4. This data was not known until the Times published its report months later, on April 20, 2018. A DHS spokesman tells the Times that the agency does not separate families at the border as a means to deter illegal immigration. The statement appears to contradict Kelly’s previous comments in which he said he was considering a family-separation proposal as part of a broader deterrence effort.

Dec. 5, 2017

The Senate votes to confirm Kirstjen Nielsen as the new Department of Homeland Security secretary. Nielsen replaces Kelly, who became President Donald Trump’s chief of staff.

April 8, 2018

Attorney General Jeff Sessions announces in early April that anyone “illegally entering this country will not be rewarded, but will instead be met with the full prosecutorial powers of the Department of Justice.” In a memo, Sessions directs U.S. attorneys offices in southwest states along the border to prosecute all cases flagged for illegal entry, “to the extent practicable.”

May 7, 2018

Following his April directive Sessions announces a “zero-tolerance” policy for illegal entry into the U.S.: “If you cross this border unlawfully, then we will prosecute you. It’s that simple,” he said. According to the policy, any migrant crossing the border beyond the official ports of entry will face criminal prosecution, including asylum seekers with children.

When asked about Sessions’ comment later that day, White House press secretary Sarah Huckabee Sanders repeatedly told reporters that family separation was happening because of existing laws, “and the president is simply enforcing them.” Again, that’s not true. There’s no law mandating that.

June 15, 2018

During a briefing call today, Homeland Security officials confirmed to reporters that 1,995 children were separated from 1,940 adults at the U.S.-Mexico border from April 19 through May 31. These adults were referred for prosecution after illegally crossing the border.

June 18, 2018

Members of the Trump administration doubled down on the separation of children from their parents at the border after a weekend of growing criticism over the practice. Homeland Security’s Nielsen said officials will not offer an apology for enforcing the country’s immigration laws.

Attorney General Sessions said if officials didn’t enforce the laws, they would “encourage hundreds of thousands of people year to likewise ignore our laws and illegally enter our country.” Trump said this was “very strongly the Democrats’ fault.” All three have pointed to a law that mandates family separation, but no such law exists.

June 20, 2018

Trump signed an executive order that aims to end the separation of children from their parents when they’re detained for trying to cross the U.S.-Mexico border illegally. The day before, House Republicans introduced proposals with a similar goal, but Democrats wouldn’t support it, saying Trump could end family separation through executive order.

The order says the government will detain migrant families together throughout “criminal proceedings for improper entry or any removal or other immigration proceedings.” But Trump added that the executive order would not end his administration’s “zero-tolerance” policy of prosecuting immigrants for crossing the border illegally.

 

Along with addressing family separation, the order directs the Secretary of Defense to make existing military facilities available to house migrant families if necessary.

The order also asks Attorney General Sessions to file a formal request to modify the “Flores agreement,” which set standards on how minors are treated while in federal custody. The DOJ has yet to make this request. Members of the Trump administration has falsely claimed that the Flores agreement, which stems from a 1980s case over the mistreatment of unaccompanied minors, forced them to separate families at the border.

June 26, 2018

The Democratic attorneys general in 17 states, including New York, California and Virginia, filed a lawsuit against the Trump administration, saying that their family separation policy is a “cruel and unlawful” practice. The lawsuit, filed with the U.S. District Court in Seattle, is believed to be the first legal challenge over Trump’s policy.

In a separate legal action, immigrant advocates requested that a federal judge in Los Angeles order the release of parents separated from their children at the border. The Justice Department did not respond to either legal action.