McALLEN, Texas (AP) — For about four months before President Joe Biden took office, advisers engaged in intense internal debate about how quickly they should undo his predecessor's hardline border policies.

The answer, almost always, was not soon enough. Immigration advocates on the transition team shot down a detailed memo circulated among top aides that called for turning back some migrants who cross illegally by making them seek protection in other countries. They pushed back against estimates of soaring migration flows if Donald Trump's policies were dismantled.

In the end, Biden recognized predictions that more migrants could come but was firm that policies instituted by Trump were inhumane and had to be jettisoned.

Almost immediately, the numbers exceeded expectations.

Keep scrolling for a gallery of images of the border under President Biden

Children traveling alone shattered previous highs in March. The Border Patrol encountered migrants in South Texas more often than ever in June and July.

In September, about 15,000 mostly Haitian refugees were camped under a bridge in the border town of Del Rio, Texas. For days, migrants waded back and forth across a river for supplies and slept in squalor. Images of agents on horseback corralling refugees went viral.

The administration veered between permissive and restrictive responses, leaving it politically isolated and underscoring consequences of not having a new asylum system in place when it rescinded Trump's policies.

Some developments could not have been predicted, and pre-dating Biden was immigration courts taking nearly four years on average to decide a case of someone not in custody.

But a close review of the last year — based on internal documents obtained by The Associated Press and AIM Media Texas and dozens of interviews with current and former U.S. and Mexican officials, among others — shows how an administration stacked with immigration advocates was unprepared for the huge increase in people seeking refuge at the border. Many interviewed spoke on condition of anonymity because they were not authorized or comfortable discussing private deliberations.

The White House defended its record when presented with specifics of the reconstruction.

"After four years of the Trump administration's chaos, cruelty and misplaced priorities, the work to build a fair, orderly and humane immigration system will take time and won't happen overnight," said spokesman Vedant Patel. "In a short period of time, the Biden administration continues to make considerable progress delivering on its plan."

FILE - Vice President Kamala Harris, left, listens as President Joe Biden delivers remarks on immigration, in the Oval Office of the White House in Washington on Feb. 2, 2021. Biden took office on Jan. 20 and almost immediately, numbers of migrants exceeded expectations. 

From the start, Biden was adamant about reversing Trump's "Remain in Mexico" policy, known officially as "Migrant Protection Protocols," under which about 70,000 asylum-seekers were made to wait outside the country for hearings in U.S. immigration court. He wanted to admit those already subject to the policy into the United States to pursue their claims. Despite daunting logistical challenges and early technical hiccups, thousands returned.

Mexico popped a surprise that severely undermined Title 42, which had been the biggest victory for enforcement-minded aides who fought during the transition to temporarily maintain it. Advocacy groups have repeatedly called to end the public health measure, which was invoked on grounds of preventing the coronavirus from spreading.

There was "great frustration and irritation" at the administration's highest levels when authorities in the state of Tamaulipas resisted taking expelled Central American families, according to one person with direct knowledge of discussions with Mexican officials. The change was linked to a child welfare law that took effect in January.

U.S. officials asked Mexico to delay it but got nowhere. They concluded Tamaulipas Gov. Francisco Garcia Cabeza de Vaca was trying to stymie President Andres Manuel Lopez Obrador, a political rival.

Biden had exempted unaccompanied children from Title 42 after a court authorized immediate expulsion. As a father and grandfather, he couldn't go forward with it.

Title 42 became largely a tool for removing single adults.

By midsummer, the pendulum swung to enforcement as patience wore thin in Biden's inner circle. Dramatic increases in migration flows bucked a pattern of declines during summer heat. The number of unaccompanied children reached a new high in July, breaking the previous record in March, when more than 4,500 mostly unaccompanied children were crammed into a holding facility built for 250 under COVID-19 guidelines.

The Rio Grande Valley came alive each night with inflatable rafts carrying families across the river.

The Border Patrol's Rio Grande Valley sector stopped more than 20,000 migrants for the first week ever from July 18-24.

FILE - Migrants, many from Haiti, wade across the Rio Grande river from Del Rio, Texas, to return to Ciudad Acuña, Mexico, Sept. 20, 2021, to avoid deportation from the U.S. The Border Patrol encountered migrants in South Texas more often than ever in June and July, dashing expectations for a common summer slowdown.

Brian Hastings, who heads the sector, wrote senior Border Patrol officials on July 26 that Catholic Charities of Rio Grande Valley was full and could no longer provide temporary shelter to migrants, which forced the agency to release thousands at a McAllen, Texas, bus station.

In September, the chaotic scene emerged in Del Rio, Texas, as thousands of mostly Haitian migrants converged in a makeshift encampment.

Homeland Security Secretary Alejandro Mayorkas said their arrival was unusually swift, but people with direct knowledge say Colombia provided intelligence that large numbers were moving toward Panama on their way to the U.S. border.

The U.S. flew about 8,000 Haitian migrants back to strife-ridden Haiti since Sept. 19, one of its swiftest, large-scale expulsions of refugees or migrants in decades. Thousands of others were allowed to remain in the U.S.

The uneven response after months of rising arrivals sparked sharp criticism from both the right and left.

The administration has taken modest steps to reform border policies, such as establishing a "rocket docket" for asylum-seeking families at the border and restoring an Obama-era program for unaccompanied children to apply in Central America to join their parents in the U.S. Its most ambitious proposal — adapted from the Migration Policy Institute think tank — is to assign asylum officers to the border to more quickly decide cases than immigration judges. Seemingly technical, it may have impact.

Despite that proposal, published in August, the administration has yet to release detailed plans of the "humane" asylum system that Biden promised during his campaign.

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PHOTO GALLERY

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This report is a collaboration between The Associated Press and AIM Media Texas, publisher of The (McAllen) Monitor and other newspapers. Spagat, a San Diego correspondent, leads AP's U.S. immigration team. Gonzalez is an investigative reporter for The Monitor.