A coalition of former FBI officials has issued a warning about a "new and imminent danger" for the U.S. at the southern border, suggesting the country has "been invaded" by military-age foreign nationals.

In a letter sent last week to congressional leaders in the House and Senate, the retired officials "express our concern about a current, specific threat that may be one of the most pernicious ever to menace the United States."

The letter warns "the United States is facing a new and imminent danger" and highlights what the officials say is a threat of an invasion at the southern border, where there were over 302,000 migrant encounters in December after a record fiscal 2023 in which there were 2.4 million migrant encounters.

"In its modern history, the U.S. has never suffered an invasion of the homeland and, yet, one is unfolding now," they say. "Military aged men from across the globe, many from countries or regions not friendly to the United States, are landing in waves on our soil by the thousands — not by splashing ashore from a ship or parachuting from a plane but rather by foot across a border that has been accurately advertised around the world as largely unprotected with ready access granted."

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The ten officials include former FBI assistant directors Kevin Brock and Chris Swecker, former Terrorist Screening Center Director Timothy Healy and former acting Customs and Border Protection Commissioner Mark Morgan, who is also a former FBI superintendent.

The lawmakers note the releases of migrants into the country in large numbers, although federal officials have repeatedly claimed that migrants are only released after going through a multi-layered vetting process and biographical and biometric vetting. 

They also note numbers of gotaways, which are estimated to have exceeded 800,000 last fiscal year, and encounters of those on the FBI’s terror watch list. FBI Director Christopher Wray has also expressed concern about who is evading Border Patrol. 

The Biden administration has, however, also said it has greater awareness of the border now and that gotaways are a decades-old problem for the country.

"With the investments that this Congress has made in the Border Patrol, CBP as a whole, we have greater situational awareness now than I’ve ever had," Border Patrol Chief Raul Ortiz said in March last year.

The former officials express concern, particularly in light of the Oct. 7 Hamas attack on Israel.

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"It is stark to say so, but having a large number of young males now within our borders who could begin attacking gatherings of unarmed citizens, in imitation of 10/7 and at the behest of a foreign terror group, must be considered a distinct possibility," they wrote. 

"We would be remiss not to call out this potentially grave threat in the most direct terms. The warning lights are blinking."

They warn that some of the men arriving to the border are from terror-linked areas as well as geopolitical foes like China and Russia.

"For these reasons, elements of this recent surge are likely no accident or coincidence. These men are potential operators in what appears to be an accelerated and strategic penetration, a soft invasion, designed to gain internal access to a country that cannot be invaded militarily in order to inflict catastrophic damage if and when enemies deem it necessary," they say.

They encourage Congress and other federal leaders to take action and pay more attention to the threat.

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"The country has been invaded, an invasion that will continue as long as the nation’s enemies perceive it will be tolerated," they say.

The letter was written at a time when border security is a top political issue. Republicans have blamed the migrant crisis on Biden-era policies and the reversal of Trump-era policies. The administration says it needs more funding and reforms to fix a "broken" system and deal with a hemisphere-wide crisis. DHS has also pointed to an increase in returns of illegal immigrants, including more since May than in the entirety of fiscal 2019.

Lawmakers are debating a supplemental spending bill that includes $14 billion for border funding for staffing, processing, removals and aid to cities and organizations. Republicans, however, want to see tightened asylum restrictions and fewer parole releases.