WASHINGTON (AP) — Photos and video of a Border Patrol agent on horseback using his long leather reins to lash at Haitian migrants along the U.S.-Mexico border prompted expressions of outrage Tuesday from Homeland Security Secretary Alejandro Mayorkas, who sought to explain away the action a day earlier during a visit to South Texas.

Mayorkas said he was “horrified” by the images that have spread widely on social media and drawn outrage from members of Congress, including House Speaker Nancy Pelosi. Some of the mounted agents use their horses to forcibly move and block the migrants, seeming to herd them like animals, and taunt them in at least one instance.

“Any mistreatment or abuse of a migrant is unacceptable,” Mayorkas said in an interview with CNN. “The pictures that I’ve observed troubled me profoundly.”

Customs and Border Protection’s Office of Professional Responsibility is investigating the incident, DHS said in a statement issued late Monday. The announcement came hours after the secretary appeared alongside Border Patrol Chief Raul Ortiz to discuss U.S. efforts to clear an encampment of thousands of Haitian migrants in and around the small city of Del Rio that has emerged suddenly in recent days.

Vice President Kamala Harris told reporters Tuesday that she would be discussing the situation with the DHS secretary. “What I saw depicted about those individuals on horseback treating human beings the way they were was horrible,” she said. “And I fully support what is happening right now, which is a thorough investigation into exactly what is going on there. But human beings should never be treated that way. And I’m deeply troubled about it.”

The controversy added an additional complication to an already difficult situation for the Biden administration, which is facing criticism from many allies for quickly sending Haitians back to their troubled homeland under a public health order without giving them an opportunity to seek asylum in the United States.

The disturbing videos and photos were taken in recent days in and around Del Rio and show Border Patrol agents confronting Haitians along the Rio Grande near a border bridge where thousands of migrants have gathered in recent days in hopes of entering the country. Videos by The Associated Press and other news organizations show one agent twirling his reins in a menacing way but not actually striking anyone.

Ortiz, asked about what some saw as the use of whips on Haitians, said the agents were working in a difficult and chaotic environment and trying to control their horses. “We do not know who are the smugglers or who are the migrants,” he told reporters, insisting that the agency was looking into what happened.

Mayorkas then stepped in to explain that the agents weren’t using whips. “To ensure control of the horse, long reins are used, but we are going to investigate the facts to ensure that the situation is as we understand it to be, and if it’s anything different we will respond accordingly.”

Video and photos viewed by The Associated Press appear to confirm that the agents were not using whips when they were encountering migrants along the banks of the Rio Grande.

The outrage built quickly, including at the White House, where press secretary Jen Psaki said the images were “horrific,” though she acknowledged not having all the information.

“I have seen some of the footage. I don’t have the full context. I can’t imagine what context would make that appropriate,” Psaki told reporters.

Criticism was withering from members of Congress, including Rep. Bennie Thompson, a Mississippi Democrat who is chairman of the House Homeland Security Committee. He called on Mayorkas to “take immediate action to hold those responsible accountable.”

Pelosi called the reports of mistreatment and “use of what appear to be whips” to be “deeply troubling” and also called for an investigation.

“All migrants seeking asylum must be treated in accordance with the law and with basic decency,” the speaker said.

A White House official, speaking on condition of anonymity to discuss internal operations, said that, according to DHS, the secretary hadn’t seen the footage until after the news conference on Monday.

Asked about the photos and video at a Senate hearing Tuesday, Mayorkas told lawmakers that the issue had been “uppermost in my mind” since he saw them, though it wasn’t clear which he had viewed.

“I was horrified to see the images, and we look forward to learning the facts that are adduced from the investigation, and we will take actions that those facts compel,” he said. “We do not tolerate any mistreatment or abuse of a migrant. Period.”

Mayorkas said he wouldn’t explicitly say whether they reflected mistreatment or abuse, deferring instead to the investigation.

The Office of Professional Responsibility is leading the investigation, and will have personnel on the ground full-time monitoring the situation in Del Rio, the agency said.

More than 6,000 Haitians and other migrants have been removed from the encampment in Del Rio, and Mayorkas predicted a “dramatic change” in the number of migrants there within the next two to four days as it continues the removal process.

The latest increase in migration at the border has prompted Republicans to renew attacks on Democrats for an increase in attempts to illegally enter the country. Mayorkas says DHS is dealing with a “heartbreaking situation” at the border, where the total number of encounters declined last month for the first time since President Joe Biden took office.

“I by no means diminish the, the humane issue that it presents, but I want to be clear that we do have a plan to address it,” he said.

Mayorkas said 600 Homeland Security employees, including from the Coast Guard, have been brought to Del Rio. He said he has asked the Defense Department for help in what may be one of the swiftest, large-scale expulsions of migrants and refugees from the United States in decades.

Speaking from Del Rio on Monday, Mayorkas warned Haitians not to come to the U.S., noting that a temporary extension in protections from deportation for Haitians was extended through July 29, those protections no longer apply to new migrants.

“If you come to the United States illegally, you will be returned. Your journey will not succeed, and you will be endangering your life and your family’s life,” he said.

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