(The Center Square) – Hundreds of immigrants on the federal terror watch list have been caught at the southern and northern U.S. borders in recent years, and national security experts say there is a real threat of a terrorist attack on American soil as a result.

With the conflict between Israel and the Gaza-based terrorist group Hamas escalating, that threat has only grown.

U.S. Customs and Border Protection’s San Diego field office warns that Hamas may attempt to send individuals to the U.S. through the southern border.

“San Diego Field Office Intelligence Unit (SDFO-FITU) assesses that individuals inspired by, or reacting to, the current Israel-Hamas conflict may attempt travel to or from the area of hostilities in the Middle East via circuitous transit across the Southwest border,” an unclassified document from the office reads.

“Foreign fighters motivated by ideology or mercenary soldiers of fortune may attempt to obfuscate travel to or from the US to or from countries in the Middle East through Mexico.”

Lora Ries, a former acting deputy chief of staff for the U.S. Department of Homeland Security, told The Center Square that there is a real threat from illegal immigrants connected to Hamas, the terror group that killed more than 1,400 in Israel and took dozens of hostages this month.

“A terrorist attack facilitated or inspired by Hamas is absolutely a real threat in the United States,” she said. “The visceral hatred toward Israel and America that’s on display here at home and around the world should be a wakeup call. Since Biden took office, millions of inadmissible aliens have flooded through the open borders and into our country from over 160 countries around the world – including those harboring terrorists.”

U.S. House Committee on Homeland Security Chairman Mark E. Green, R–Tenn., raised similar concerns, pointing to federal data showing the high number of terrorism-connected migrants caught at the both the northern and southern border.

“Every American should be concerned about the state of our homeland security under this administration,” Green told The Center Square. “President [Joe] Biden and Secretary [Alejandro] Mayorkas’ refusal to properly enforce the laws passed by Congress or provide our frontline law enforcement with the policies they need to accomplish their mission has empowered our nation’s enemies.”

According to federal data, 736 known or suspected terrorists were apprehended at U.S. border in fiscal year 2023 alone, the most ever recorded according to available data. In recent years, there have been thousands of these migrants on the Terrorist Screening Center list.

Those figures do not account for migrants who were undetected or evaded capture from border patrol agents.

“How many known or suspected terrorists have successfully evaded apprehension alongside the nearly 1.6 million known gotaways at the Southwest border?” Green said. “And how many more do we have to apprehend at the border before Secretary Mayorkas feels it warrants real action? Communities across the country are in danger because of Secretary Mayorkas’ dereliction of duty. It is beyond unacceptable that Congress is still waiting for full transparency on these threats.”

These terror-connected migrants are part of a flood of millions of illegal migrants since Biden took office.

The increased illegal immigration has put increasing pressure on the Biden administration, which announced earlier this month that it would restart construction on a small Texas portion of the southern border wall, a Trump administration project that was largely derided by Democrats.

A group of Democratic state and local elected leaders sent a letter to the White House earlier this month calling on Biden to address the border crisis, which is costing states and municipalities billions of dollars to address.

The FBI maintains the Terrorist Screening Center, a multi-agency effort created after the 9/11 attacks to centralize one list of whom to watch across federal agencies.

When asked, the FBI gave few answers, deferring to the Department of Homeland Security.

“Specific intelligence-related criteria must be met for a person to be added to the TSC watchlist,” an FBI spokesperson told The Center Square.

A DHS spokesperson told The Center Square that the increased immigration from South America is a key cause for the increase in suspected terrorists but that the agency is focused on the problem.

They also said these encounters with suspected terrorists still represent a fraction of one percent of the overall encounters. In addition, sometimes family members or “known affiliates” of suspected terrorists could be counted on the watch list as well.

“As we learned on 9/11, it only takes a handful to destroy lives,” Ries, who now works with the Heritage Foundation, told The Center Square. “Meanwhile, Biden and his allies are burying their heads in the sand, refusing to secure our border or admit their progressive policies leave us open to attack. Americans should go about their daily lives as usual but add a layer of vigilance given the global atmosphere, dangerous lack of immigration enforcement, and clear anti-American sentiment that has grown in the U.S. after years of demonization from the Left.”