FIRST ON FOX: Sen. James Lankford, R-Okla., is seeking answers from the head of Customs and Border Protection (CBP) over the controversial investigation into Border Patrol agents who attempted to stop Haitian migrants crossing into the U.S. while on horseback -- and were falsely accused of whipping them by President Biden.

"I write to you today to inquire about due process and impartiality -- to which [CBP] personnel are entitled -- in relation to the disciplinary action being proposed against U.S. Border Patrol agents following the Horse Patrol Unit activity in Del Rio, Texas on Sept. 19, 2021," Lankford says in a letter, obtained by Fox News Digital, to CBP Commissioner Chris Magnus.

The agents were investigated after photos emerged of them encountering migrants in Del Rio during the Haitian migrant crisis, where thousands of Haitians were seeking to enter the U.S. The agents used their reins to control the horses -- but that was falsely interpreted by media outlets and top Democrats as the agents using whips against migrants.

That false claim, quickly debunked, was still echoed by President Biden -- who promised the agents would "pay."

CBP FINDS ‘NO EVIDENCE’ BORDER PATROL AGENTS WHIPPING HAITIAN MIGRANTS, BUT STILL SEEKS TO DISCIPLINE THEM 

"To see people treated like they did, horses barely running over, people being strapped — it's outrageous," Biden told reporters, making a whipping motion with his hand. "I promise you, those people will pay. There will be an investigation underway now and there will be consequences. There will be consequences."

Biden’s comments led to widespread concerns, including among Border Patrol, that investigators were now under pressure to find some infraction to punish them -- even if the whipping claim was bogus.

Oct 19, 2021: Senator James Lankford speaks during a Senate Finance Committee hearing on the nomination of Chris Magnus to be the next U.S. Customs and Border Protection commissioner. 

Oct 19, 2021: Senator James Lankford speaks during a Senate Finance Committee hearing on the nomination of Chris Magnus to be the next U.S. Customs and Border Protection commissioner.  (Mandel Ngan/Pool via REUTERS)

CBP released its report earlier this month, which concluded there was "no evidence" that the agents struck anyone with their reins, or that they even carried whips.

Magnus announced the findings at a press conference, where he said that the agents were still to face disciplinary actions for "offensive" language, using an "unmoderated" radio and of having maneuvered a horse around a child in an "unsafe manner."

Magnus said the process was incomplete, as the employees have been given the materials and the right to reply. He said he would also not be providing specifics of the disciplinary proposals. He also rejected the idea that there was any political pressure on the Office of Professional Responsibility (OPR).

"It is important that I am clear that OPR, and its investigations, are insulated from influence within and outside CBP, including the Department of Homeland Security. This means they do not even tell me what they are doing and I certainly do not tell them how to do their job," he said.

BORDER PATROL AGENTS FUME AS DHS PREPARES TO PUNISH OFFICERS CAUGHT UP IN ‘WHIP’ CONTROVERSY: ‘BULL----’

But those comments have done little to assuage concern from Republicans of a politically-motivated investigation.

"I am deeply concerned about the appearance of partiality, and, consequently, the administration’s potential failure to provide due process during both the investigation and the disciplinary process resulting from this event," Lankford said in his letter.

Additionally, Lankford questioned why Magnus would hold a press conference on the matter when the process had not fully been completed.

"Specifically, I find it troubling that you appeared in a press conference following the final report findings where you discussed the actions which will be taken against CBP officials," he said. "These are proposed actions and the process due to these officials has not yet been completed."

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"The partiality and politicization administration officials have generated around this incident has placed undue pressure on agents, the agency, and agency officials involved in the investigation and disciplinary process," he says.

Lankford asks Magnus for a number of details about the circumstances of the investigation, including what role he or any politically appointed staff had in the development of the OPR report, if they received a draft and if so, if they recommended any changes.

He also asks whose decision it was to hold the press conference, and if any officials at CBP or DHS raised concerns about holding such a press conference.