Photos and video of a US Border Patrol agent on horseback using his long leather reins to lash at Haitian migrants along the US-Mexico border prompted outrage on Tuesday from Homeland Security Secretary Alejandro Mayorkas.
Mr Mayorkas said he was “horrified” by the images that have spread widely on social media and drawn condemnation 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,” Mr Mayorkas said in an interview with CNN. “The pictures that I’ve observed troubled me profoundly.”
The US Department of Homeland Security (DHS) is investigating the incident along the Rio Grande River near Del Rio, Texas, where up to 12,000 migrants had been camping at one point.
US Vice President Kamala Harris on Tuesday said that she fully supports the investigation.
"Human beings should never be treated that way and I am deeply troubled by it," Ms Harris said.
White House Press Secretary Jen Psaki on Monday said that US policy is to treat people “humanely” and that she could not imagine a scenario that would make the context of the footage in question acceptable or appropriate.
DHS on Monday said it does not “tolerate the abuse of migrants in our custody and we take these allegations very seriously”.
The statement added that the US Customs and Border Protection Office of Professional Responsibility is investigating the matter.
The US in recent days has begun a mass expulsion of Haitian migrants, with authorities expected to remove many of the 12,000 people in Del Rio who had crossed from Ciudad Acuna in Mexico.
The first flight carrying migrants arrived in the Haitian capital of Port-au-Prince on Sunday. Mr Mayorkas said he expects four more flights to leave on Tuesday.
US authorities have moved 3,300 migrants since last week and they aim to “quickly” process the thousands more living underneath the bridge that connects Del Rio with Ciudad Acuna.
Mr Mayorkas had previously warned migrants to abandon their trek, saying the government would have “no choice” but to expel them. Many Haitians had travelled from as far south as Chile, hoping to apply for asylum in the US.
“If you come to the US illegally, you will be returned,” Mr Mayorkas said at a news conference on Monday. “Your journey will not succeed.”
A White House official said on Monday night that the Biden administration is responding to the crisis by trying to provide support to migrants at the border and flying them back to Haiti under Title 42, a Trump-era immigration policy that permits the expulsion of migrants.
While many migrants returned to Mexico to avoid being deported, some remain undeterred, fearing what awaits them if they are forced to return to their country.
One migrant, travelling with his wife and 2-year-old daughter, said they could not return to Haiti because of July's presidential assassination. The country was also struck by a devastating earthquake last month that left more than 2,200 dead and destroyed about 6,100 homes.
Ms Harris on Tuesday acknowledged the multiple tragedies that have befallen Haiti this summer, and that the US must do better to support their basic needs.
"People want to stay home, they don't want to leave home. But they leave when they can't satisfy their basic needs," Ms Harris said.
US Secretary of State Antony Blinken on Monday night said he had spoken with Haitian Prime Minister Ariel Henry “about co-operation to repatriate Haitian migrants on the southern border of the United States”.
Mexican immigration officials on Saturday announced they have begun a dialogue with Haiti to address the flow of migrants.