Living in the United States without legal authorization is a civil offense, handled by immigration courts that can issue deportation orders. But crossing the border illegally is a misdemeanor, punishable by up to six months in prison. And anyone with a removal on their record can be prosecuted for the felony of illegal reentry and imprisoned for up to two years, if they have an otherwise clean criminal record, and up to 20 for those convicted of serious crimes.

Congress first passed those laws in 1929, at the urging of a notorious segregationist senator from South Carolina, but invoked them only sporadically until around 2005, when immigration authorities wanted to beef up penalties for jumping the border to deter migrants. Prosecutions have become an increasingly controversial part of the immigration enforcement system, and have accounted for around half of the federal criminal caseload since 2009.

These prosecutions were a key feature of former President Donald Trump’s hardline approach to immigration. In 2018, then-Attorney General Jeff Sessions announced a “zero tolerance” policy requiring all U.S. attorneys in the five districts that touch the U.S.-Mexico border to systematically prosecute every unauthorized migrant Border Patrol apprehended. The family separation debacle that year resulted from the Trump administration extending zero tolerance to parents traveling with their kids, shunting mothers and fathers into criminal courts while their kids remained in civil custody.

With less publicity, Sessions also directed U.S. attorneys in all 94 federal districts to consider illegal reentry prosecutions for any migrants with a prior removal on their record, regardless of their criminal records, ties to the community or fears of returning to their home countries.

Several of the 2020 Democratic presidential candidates, led by Julián Castro, championed repealing the laws criminalizing illegal entry in response to Trump’s family separations. Though Biden wasn’t among them, some defenders imagined his administration might begin to drop at least some of the most egregious immigration prosecutions, as Obama’s had done in immigration court.

Their wish partially came true. Shortly after Biden took office, defenders outside the southern border region stopped seeing Trump-era numbers of otherwise petty immigration offenders getting charged with felony reentry.

But months after Trump’s exit from the White House, cases still linger in federal court as the Biden administration inherits the remnants of zero tolerance with no apparent interest in reversing it.