The Trump administration has carried out nearly 10,000 summary deportations or “expulsions” since March 21, using emergency public health measures during the coronavirus outbreak that have given U.S. Customs and Border Protection broad authority to bypass immigration laws, CBP officials said Thursday.

The measures have allowed the agency to quickly turn away most unauthorized migrants — sending them back across the U.S.-Mexico border. The moves have dramatically slashed the number of detainees held in border stations, where they fear the coronavirus could spread, the officials said. CBP has fewer than 100 detainees in custody, down from nearly 20,000 at this time last year during the border crisis, officials said.

Since the implementation of the rapid expulsions, migration levels have fallen to near their lowest point in decades, with unlawful border crossings down 56 percent, said acting CBP commissioner Mark Morgan. He also acknowledged that the United States has all but closed its borders to asylum seekers who are fleeing persecution, including those who attempt to enter legally at U.S. ports of entry.

“Those who are undocumented or don’t have documents or authorization are turned away,” Morgan said.

Democratic lawmakers have accused the administration of defying U.S. laws and exceeding the authority of the coronavirus public health emergency order, but Morgan defended the emergency measures as a necessary step to stop the spread of the disease.

“This is not about immigration,” Morgan said. “This is about public health. This is about putting forth aggressive mitigation and containment strategies.”

Morgan and other Trump administration officials say the clampdown at the border is an urgent necessity at a time when much of the country is effectively locked down, with all but eight states under broad stay-at-home orders. The established U.S. enforcement model at the border — apprehend, detain, prosecute, deport — is too much of a liability during a pandemic, they say.

The sweeping measures have replaced safeguards and immigration laws President Trump calls “the worst in the world” with the kind of strict model he has long extolled as a solution to the country’s border challenges.

With the pandemic consuming the country’s attention, the administration also has waived environmental regulations and moved to weaken the Affordable Care Act, and Trump himself has fired inspectors general, pushed tax cuts and urged voting restrictions.

“What is happening at the border right now is a tragedy,” said Kari Hong, an immigration attorney who teaches at Boston College Law School. “We are abandoning our legal commitment to provide asylum to people whose lives are in danger in other countries.”

Hong said the United States government is exploiting the coronavirus crisis to achieve a hard-line anti-immigration agenda: “By invoking these emergency orders, the Trump administration is simply doing what it’s wanted to do all along, which is to end asylum law in its entirety.”

CBP said the number of migrants taken into custody at the border fell to 33,937 in March, down 7 percent from February. Single adults from Mexico accounted for 70 percent to 75 percent of those detained, and most of the remainder were from Central America’s Northern Triangle countries: Guatemala, El Salvador and Honduras.

The Mexican government has agreed to accept the rapid return of migrants from those nations at the border under an agreement reached with the Trump administration last month.

The recent expulsions include children who would otherwise be protected from rapid removal by U.S. anti-trafficking laws. Since the emergency order took effect last month, the United States has expelled nearly 400 underage migrants, according to the most recent tally by Reuters. The minors were released into Mexico or boarded onto planes and flown to Central America without being transferred to the care of the U.S. Department of Health and Human Services for their safety, as would be typical.

In a letter Wednesday to acting Homeland Security secretary Chad Wolf, Sen. Patrick J. Leahy (D-Vt.) and other Democrats on the Senate Judiciary Committee said the Trump administration’s use of a 1944 health emergency law to suspend due process at the border is not legal.

“Contrary to existing law, individuals, families, and children are now unable to sufficiently make claims for asylum, seek other forms of humanitarian protection, and, in some instances, are being expelled to countries in which they fear persecution,” the senators wrote.

“DHS has essentially determined that executive branch officials can all but ignore the long-standing federal laws pursuant to an executive branch interpretation of a law enacted in 1944,” they said. “This amounts to a startling expansion of executive power under the guise of a global pandemic response.”

The falling number of migrants in CBP custody has led to sharp drop in the number of migrants transferred to the network of U.S. Immigration and Customs Enforcement jails around the country. The number of detainees in ICE facilities — where fears of a major outbreak remain high — has dropped to about 34,000 nationwide, according to the latest figures. That is the lowest number for the ICE jail population since 2016.

ICE has had 37 confirmed cases of the coronavirus among detainees in its custody. Sixteen of the cases have been in facilities in New Jersey, and nine have been discovered in the Otay Mesa Detention Center in San Diego. Cases also have been confirmed at ICE facilities in Arizona, Louisiana, Pennsylvania, Michigan and Florida.

ICE officials have asked field office directors to review whether there are detainees over 60 who have certain health conditions that put them at higher risk of infection so they can evaluate potential release, according to ICE guidance distributed Saturday and filed as part of a legal case in the U.S. District Court for the District of Columbia. “The fact that an alien is potentially higher-risk for serious illness from COVID-19, may form the basis for a determination that ‘continued detention is not in the public interest,’ ” and could justify release, the guidance says.

Severe overcrowding in CBP facilities last year led the agency to hold tens of thousands of migrants in tent facilities and chain-link enclosures, releasing many into the interior of the United States under procedures Trump decries as “catch and release.”

The average amount of time it takes agents to expel border-crossers to Mexico under the emergency protocols is 96 minutes. The agents record migrants’ personal data and biometric info, and then load them into vans for rapid return to the nearest border crossing. No medical exams are performed unless the person is in distress.

Morgan said migrants who state a fear of harm if sent back to Mexico are considered for an exemption from the public health order on “a case-by-case basis.” According to an internal memo obtained by ProPublica, border agents need the approval of supervisors to consider such an exemption.

Spencer S. Hsu contributed to this report.