Migrants, who were caught trying to sneak into the U.S., are led by an U.S. Customs and Border Protection agent, second from left, at the McAllen-Hidalgo International Bridge while being deported to Reynosa, Mexico, Thursday, March 18, 2021, in Hidalgo, Texas. A surge of migrants on the Southwest border has the Biden administration on the defensive. The head of Homeland Security acknowledged the severity of the problem Tuesday but insisted it's under control and said he won't revive a Trump-era practice of immediately expelling teens and children. (AP Photo/Julio Cortez)

(CNN) The US is on track to encounter more than 2 million migrants at the US-Mexico border by the end of the fiscal year, according to internal government estimates reviewed by CNN, marking a record high.

The projections could be subject to change in the event of policy modifications or other changes in Latin America. US Border Patrol encounters are also expected to be largely made up of single adults, who are being turned away at the US southern border as soon as they’re encountered under a public health order, and as a result, might also account for repeat crossers.

But combined, the data — based on preliminary reporting as of this month — illustrates the continuing challenge for the Biden administration, which has already faced a series of hurdles on the US-Mexico border, particularly with unaccompanied children and migrant families. 

Data reviewed by CNN shows that up to 1.1 million single adults are expected through September, along with up to around 828,000 families and more than 200,000 unaccompanied children. Border Patrol encounters are expected to continue to rise month-by-month, according to the projections, which can vary.

The last time Border Patrol apprehensions surpassed 1 million was in fiscal year 2006, according to publicly available data from Customs and Border Protection. Border Patrol arrests also climbed during the 2019 border crisis, but fell short of 900,000.

Deputy Chief of the Border Patrol Raul Ortiz told reporters Tuesday that the agency expects to encounter more than a million migrants this fiscal year.

Homeland Security Secretary Alejandro Mayorkas also warned of the uptick in mid-March, saying: “We are on pace to encounter more individuals on the southwest border than we have in the last 20 years. We are expelling most single adults and families. We are not expelling unaccompanied children.”

CNN reached out to the Department of Homeland Security. Customs and Border Protection pointed to Ortiz’s comments.

It’s difficult to compare today’s situation with those of recent years because of the drastically different circumstances, in part relating to the coronavirus pandemic. There are several factors at play — including deteriorating conditions in Latin America, pent-up demand to enter the US and a perceived relaxation of enforcement under President Joe Biden — that are driving migrants to the border at what appears to be an accelerated pace.

The administration is also relying on a public health law known as Title 42 that was invoked under former President Donald Trump and allows border authorities to turn back migrants encountered at the US-Mexico border, either to Mexico or their country of origin. Unaccompanied children are not subject to the policy.

These projections could also include a high number of people crossing the border twice or multiple times. In February, around 25% of people encountered at the border had crossed more than once, up from 7% for all fiscal year 2019, the most recent annual data available.

“We are seeing higher than usual recidivism rates, as a result of COVID protocols. So the number of encounters, while they impact our operations at Border Patrol, they also can seem to overstate the migrant flows that we are seeing,” a CBP official said earlier this month.

In his first news conference, Biden noted that fluctuations in migration flows are common.

“It happens every single, solitary year: There is a significant increase in the number of people coming to the border in the winter months of January, February, March. That happens every year,” he said.

Biden also said his administration is in discussions with Mexico to have the country receive migrant families expelled by the US under the pandemic-related policy, indicating the pressure the US is placing on Mexico to help stem the flow of migration to the US.

“Mexico is refusing to take them back. They’re saying they won’t take them back — not all of them,” Biden said, when asked why some families weren’t being returned. “We’re in negotiations with the President of Mexico. I think we’re going to see that change. They should all be going back. All be going back. The only people we’re not going to let sitting there on the other side of the Rio Grande by themselves with no help are children.”

Families from Mexico and the Northern Triangle countries are sent back to Mexico unless the country does not have the capacity to receive them, the Department of Homeland Security said in a statement this month. If Mexico won’t take the families, they are processed into the US.

Measures taken in conjunction with Mexico and other Latin American countries could help the administration as it faces an ongoing influx of migrants. But it’s still likely to be a pressing challenge for officials, as evidenced by the growing number of children.

Over recent days, the Health and Human Services Department, which is charged with the care of migrant children, announced a string of new facilities, leaning on convention centers, military sites and influx shelters to accommodate children.

But even as those beds come online, more children are being encountered daily along the border, leading to overcrowding in border facilities.

“What bogs us down is the fact that we’re having to take care of 1,200 kids. We’re already done. We already completed the Border Patrol process, so if HHS would be able to take these kids off of our hands then it would be better for everybody,” acting Executive Officer for Rio Grande Valley Operational Programs Division Oscar Escamilla told reporters during a tour of a temporary border facility Tuesday.

“We’re not in the business of dentition. We’re forced into the business because we can’t turn them over to anybody,” he added.

CNN’s Geneva Sands contributed to this report.