Former U.S. Border Patrol Chief Rodney Scott sent a letter to Senate leadership warning that the agency is losing its ability to police the southern border amid an unprecedented surge in illegal immigration.

Scott, who served in Border Patrol for nearly 30 years before stepping down in June, also warned the situation is a national security threat, according to his Sept. 11 letter addressed to Senate Majority Leader Chuck Schumer, D-N.Y. and Senate Minority Leader Mitch McConnell, R-Ky., as well as Sens. Gary Peters, D-Mich., and Rob Portman, R-Ohio.

"As Chief, I was the most senior official responsible for border security between ports of entry," Scott wrote in part. "I witnessed the [unprecedented] seismic shift in border security and immigration policy that was initiated on January 20, 2021."

BORDER PATROL CHIEF STEPPING DOWN AFTER BIDEN ADMINISTRATION MOVES TO REPLACE HIM

"I believe this policy shift and the associated public statements created the current border crisis. Of greater concern, I also witnessed a lack of any meaningful effort to secure our borders. Contrary to the current rhetoric, this is not simply another immigration surge. This is a national security threat," he continued.

Contrary to the current rhetoric, this is not simply another immigration surge. This is a national security threat.

— Retired Border Patrol Chief Rodney Scott

Scott went on to say he has been "sickened by the avoidable and rapid disintegration of what was arguably the most effective border security in our Nation's history." He blamed the deteriorating conditions on inexperienced political appointees in the Biden administration, who he claimed are ignoring the advice of seasoned professionals and giving false information to Congress and the public.

SAN DIEGO, CA - APRIL 25: Rodney Scott, chief of the Border Patrol's San Diego sector, stands for a portrait near the border wall on April 25, 2018 in San Diego, California.  (Photo by Carolyn Van Houten/The Washington Post via Getty Images)

"In my professional assessment, the U.S. Border Control is rapidly losing the situational awareness required to know who or what is entering our Homeland," Scott warned. "The ability of USBP to detect and interdict those that want to evade apprehension is being degraded daily."

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Warning that the porous border is unsustainable and that transnational criminal organizations are taking advantage of it, Scott urged congressional leadership to convene a hearing to assess the extent of the threat.

U.S. Customs and Border Protection, the agency which oversees U.S. Border Patrol, declined to comment.

Sen. Ron Johnson, R-Wisc., whose office provided Fox News with a copy of the letter, said: "CBP Chief Scott is a career, non-partisan civil servant who worked tirelessly to secure our borders and keep America safe. That he felt compelled to write this letter, which warns of the significant national security risks this administration is creating, should concern every American. I hope the Biden administration and Congress will heed his fervent warning."

Homeland Security Secretary Alejandro Mayorkas said Sunday that the administration was moving to address the most recent challenge, thousands of Haitian immigrants huddled in the outdoors after trekking across the border.

"We certainly are experiencing a challenging situation, but we are surging resources and we have a multipronged approach to this," Mayorkas told CNN on Sunday. He said that so far the administration had added 600 agents as part of a surge in the region and was working with American Red Cross and World Central Kitchen to bring supplies and food to people.