The Border Patrol Union on Tuesday said that the Biden administration "must accept responsibility" for the dozens of migrants killed in a trailer in San Antonio, Texas, as part of what is believed to be a human smuggling operation -- blaming the administration’s policies for causing "chaos" at the border.

"Add 46 to the Biden/Harris/Mayorkas/Garland death toll. Not one word from any of them about enforcing our laws & securing our border," the National Border Patrol Council said in a tweet. "This administration & its policies are the single biggest factor driving the criminal chaos on the border, and they must accept responsibility."

TEXAS GOV. ABBOTT LASHES OUT AFTER 46 MIGRANTS FOUND DEAD IN TRACTOR TRAILER 

At least 46 migrants were found dead in the 18-wheeler, which contained up to 100 migrants and was discovered abandoned as part of the suspected border smuggling operation. Dozens were hospitalized. On Tuesday, Mexican President Andres Manuel Lopez Obrador said the death toll stood at 50.

Authorities said a city worker heard a cry for help from the truck just before 6 p.m. on Monday. Victims include at least 22 from Mexico, seven from Guatemala and two from Honduras.

San Antonio Police Department Chief William McManus said a nearby worker notified authorities about the tractor-trailer shortly before 6 p.m. Upon arriving at the scene, officials found dozens of deceased inside the back of the transport vehicle.

TEXAS-MEXICO BORDER CHAOS: AT LEAST 46 MIGRANTS FOUND DEAD IN SAN ANTONIO INSIDE 18-WHEELER, REPORTS SAY

"This is now a federal investigation," McManus said. The police chief also said at least three people were in custody but did not specify if the individuals were suspects in the investigation.

"This is nothing short of a horrific human tragedy," San Antonio Mayor Ron Nirenberg said Monday night near the location where the tractor-trailer was found.

Texas Migrant Deaths trailer

Police and other first responders work the scene where officials say dozens of people have been found dead and multiple others were taken to hospitals with heat-related illnesses after a semitrailer containing suspected migrants was found, Monday, June 27, 2022, in San Antonio. (AP Photo/Eric Gay)

As the grim news emerged from Texas, it was not just the Border Patrol Union that tied the tragedy to the policies of the administration -- under which migrant encounters have skyrocketed to historic highs and more than 239,000 in May alone. Those numbers are expected to keep rising amid the summer months, even as the intense heat increases the risks to migrants making the dangerous trek north.

"These deaths are on Biden," Texas Gov. Greg Abbott tweeted. "They are a result of his deadly open border policies. They show the deadly consequences of his refusal to enforce the law."

Republicans have blamed the border surge on the administration’s rollback of key Trump-era policies such as the "Remain-in-Mexico" policy, as well as ending border wall construction, increasing the release of migrants into the interior and narrowing Immigration and Customs Enforcement (ICE) arrests and deportations. The administration has also sought to end expulsions under the Title 42 public health order – but has so far been blocked by a federal judge. 

"This tragic loss of life can be attributed to the Biden Admin open borders policies," former Homeland Security Secretary Chad Wolf said. "Nothing is compassionate about policies encouraging smugglers and traffickers to treat people like cargo. We need to enforce our laws to prevent these tragedies."

Others have called for DHS Secretary Alejandro Mayorkas to resign over the incident.

"It’s long past time for @SecMayorkas to resign," Rep. Chip Roy, R-Texas, tweeted. " If he will not, the House must impeach.  Americans are dying.  Migrants are dying.  Cartels are being enriched.  Enough."

".@SecMayorkas must RESIGN due to loss of confidence in his ability to command," Rep. Tony Gonzales tweeted.

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However, the White House pushed back on that criticism on Tuesday morning. White House Press Secretary Karine Jean-Pierre said the administration is focused on the victims and holding the human smugglers accountable -- noting a recent anti-smuggling campaign that resulted in over 1,800 arrests in two months. She also denied the claim that the border is open.

"But the fact of the matter is the border is closed, which is in part why you see people trying to make this dangerous journey using smuggling networks," she said. 

Fox News' Lawrence Richard, Griff Jenkins, Louis Casiano and The Associated Press contributed to this report.