U.S. Border Patrol personnel on the scene where dramatic images showed agents on horseback confronting groups of mostly Haitian migrants are being interviewed this week as part of an internal investigation, a law enforcement official and another source familiar with the matter told ABC News.
U.S. Customs and Border Protection launched the review following public outcry over the appearance that agents were using excessively aggressive tactics against vulnerable migrants.
It's unclear how many subjects of the investigation have been identified at this point, but the law enforcement official who spoke to ABC News said most of the interviews scheduled this week involve potential witnesses.
At least two agents were placed on administrative duties following the incident. CBP confirmed the agents involved remain on administrative detail and the horse patrol unit is still suspended from operating in the city of Del Rio, which includes a narrow strip of the international bridge where a Haitian group gathered in September.
Progress updates on any related investigation have largely been kept to a small group of principals. Some at the highest ranks of CBP are out of the loop with the goal of ensuring an independent process.
President Joe Biden gave his first reaction to the Del Rio horse patrol images when prompted by ABC News' Congressional Correspondent Rachel Scott days after the incident.
"It's outrageous, I promise you, those people will pay," Biden said. "They will be -- there is an investigation underway now, and there will be consequences. There will be consequences."
White House press secretary Jen Psaki attempted to later clarify the president's remarks, noting the White House would wait for a complete investigation.
"The president was not prejudging the outcome of an investigation," Psaki said in September. "The president was responding from his heart and responding to seeing horrific photos that we have seen over the last several days."
Biden has repeatedly spoken out publicly despite his own administration's efforts to handle sensitive matters more subtly.
Last month, Biden said those who refuse House subpoenas for the investigation into the Jan. 6 riot should be prosecuted despite prior deflections from Psaki, who cited a need for Justice Department independence.
After reports surfaced of settlement payouts of up to $450,000 per person to migrant families who had their children separated from them by the Trump administration during its use of a "zero tolerance" prosecutions at the border, Biden said it's "not going to happen" before putting an unspecified price cap on the payments.
Homeland Security Secretary Alejandro Mayorkas also said he was "horrified" by the images from Del Rio while underscoring the need to avoid conclusions before learning the results of the investigation.
Mayorkas later interrupted a reporter's question at a September press briefing in Del Río after the journalist's question included what Mayorkas characterized as unverified information.
"You are assuming the facts -- if I may say, respectfully -- you are assuming facts that have not yet been determined," Mayorkas said before turning to Border Patrol Chief Raul Ortiz, who defended the horse patrol generally.