The changes will be introduced at San Diego’s San Ysidro border crossing, according to a U.S. official familiar with the plan who spoke on condition of anonymity Thursday because it was not yet publicly announced. San Ysidro is the nation’s busiest crossing and the choice of asylum seekers who arrived to Tijuana, Mexico, in November in a caravan of more than 6,000 mostly Central American migrants.

The policy, which is expected to face a legal challenge, may be expanded to other crossings. It does not apply to children traveling alone or to asylum seekers from Mexico.

The details were finalized during bilateral talks in Mexico City over the last few days. It calls for U.S. authorities to bus asylum seekers back and forth to the border for court hearings in downtown San Diego, including an initial appearance within 45 days.