United States officials are sounding the alarm regarding the threat of terror-backed sleeper cells within the country just hours after President Donald Trump successfully ordered the elimination of key nuclear facilities within Iran.Just hours after Trump addressed the nation on U.S. military strikes on the Iranian nuclear sites, the DHS released a new memo sounding the alarm to a "heightened threat environment in the United States."The bulletin from the National Terrorism Advisory System did not cite any specific threats but comes as Iranian officials have threatened retaliation.TRUMP ADDRESSES NATION ON 'SPECTACULAR MILITARY SUCCESS' OF US STRIKES ON IRANIAN NUCLEAR FACILITIESWith the regime's nuclear program severely set back, and its infrastructure bombarded during Israel's operation, one former FBI boss told Fox News Digital that Iran and its proxies could resort to their original tactic -- terrorism. And the border crisis under the Biden administration only exacerbates the threat of terror cells in the U.S. activating to harm Americans at home."The likelihood of violent extremists in the Homeland independently mobilizing to violence in response to the conflict would likely increase if the Iranian leadership issued a religious ruling calling for retaliatory violence against targets in the Homeland," the bulletin states."It is our duty to keep the nation safe and informed, especially during times of conflict," the Department of Homeland Security said in a statement to Fox News Digital. "The ongoing Israel-Iran conflict brings the possibility of increased threat to the homeland in the form of possible cyberattacks, acts of violence, and antisemitic hate crimes."The bulletin points to law enforcement within the U.S. disrupting "multiple potentially lethal Iranian-backed plots" since 2020, while noting the Iranian governments unsuccessful attempts to target critics of its regime.MISDIRECTION AND 'DECEPTION' LIKELY KEY IN TRUMP ADMINS SURPRISE IRAN STRIKEIran previously vowed to retaliate if the U.S. became involved in Israels attack, which began after Israeli airstrikes targeted Iranian military and nuclear facilities on June 13. Prior to the attack, Trump issued a warning to Iran against retaliating, while encouraging the countrys leaders to come to a peace deal with Israel. Immediately following the strikes, Trump vowed to target additional sites within Iran "if peace does not come quickly."Theadvisory comes after Trump ordered military strikes on Irans key nuclear facilities in what officials are calling "Operation Midnight Hammer," prompting law enforcement to be on high alert regarding the threat of foreign adversary operatives conducting an attack on U.S. soil, with thousands of Iranian nationals previously crossing over the border."Because of the open borders, we are at a serious catch-up phase" Former FBI assistant director Chris Swecker told Fox News Digital. "We don't know where those thousand Iranians are and who knows how many others got across the border. We missed an opportunity when they caught and released those thousand. We missed the opportunity to gather intel by interviewing them and thoroughly vetting them. We just simply let them go, which is gross negligence on the part of the Biden administration."U.S. Customs and Border Protection did not immediately respond to Fox News Digital's request for comment.IRAN THREATENS TO HIT US BASES IN THE MIDDLE EAST: WHAT IS THE THREAT LEVEL?Over 1,200 Iranian nationals reportedly entered the U.S. during former President Joe Bidens time in office, White House border czar Tom Homan told Maria Bartiromo on "Sunday Morning Futures." The threat of foreign nationals flowing into the U.S. raises concerns regarding possible domestic sleeper cells driven by foreign terrorist organizations, according to national security experts."The challenge that the United States has had after a very long period of time, [is that] we had an open border," Michael Balboni, former Homeland Security advisor for New York state and a cybersecurity expert, told Fox News Digital. "Tens of millions of people came across and we don't know who they are, where they came from, what their capabilities are or their intentions."Swecker indicated the FBI has primarily focused its attention on homegrown attackers as foreign adversaries flowed into the country.ISRAELS ACTIONS AGAINST IRAN CREATE STRATEGIC OPPORTUNITY FOR US IN NUCLEAR TALKS, EXPERTS SAY"There's a certain population of Iranians in the U.S. that we know about, but there's thousands and thousands that we don't know about because of the open border situation," Swecker said. "The Bureau was concentrating their resources on so-called domestic terrorists like white supremacists and right-wing extremists, and calling them the most serious terrorist threat to this country when it was absolutely the international terrorists that should have been the focus of the bureau's efforts."The FBI declined Fox News Digitals request for comment.The possibility of foreign cells carrying out a domestic terror plot was pushed into the national spotlight last year after federal prosecutors announced a member of Irans Islamic Revolutionary Guard Corps and two U.S.-nationals were arrested for allegedly plotting to murder then-President-elect Trump and a U.S. citizen critical of the Iranian regime.SATELLITE IMAGE SHOWS FORDOW NUCLEAR FACILITY AFTER MASSIVE BOMB STRIKE"The Iranians have shown a pretty good capability in terms of their intelligence apparatus and their operatives being able to work clandestinely," Balboni said. "They are fairly sophisticated when it comes to that, very determined. The question is how many are in the country and the answer is we don't know."The U.S. possesses a wide range of attractive targets, with national security concerns spiking after Trump ordered the assassination of Iranian General Qasem Soleimani in January 2020."Theyre not going to quit," Swecker said. "They will never quit. And this was the right thing to do to take out their nuclear capabilities."EXPERTS CONFIDENT IRAN'S NUCLEAR PROGRAM IS NO LONGER AFTER MASSIVE US STRIKEAdditionally, critical infrastructure within major cities remains a prime spot for cells to carry out destructive terror plots, with Iranian officials likely turning to proxy groups to sow fear within the population."Electrical grids, perhaps medical facilities, transportation hubs," Balboni said. "Those types of things that have been a part of the playbook when you have these insurgencies that happened within the nation."Balboni points to instances of terror cells conducting pre-operational surveillance within the U.S., adding satellite mapping websites provide adversaries with a birds-eye view of key infrastructure.A FULL BREAKDOWN OF OPERATION MIDNIGHT HAMMER, THE LARGEST B-2 OPERATIONAL STRIKE IN US HISTORY"Law enforcement and security personnel are really focused now on trying to prevent anything from happening," Balboni told Fox News Digital. "But the problem is if you've had tens of millions of people come across the border, you really don't know where they are and what their capabilities are."As tensions between the Middle East and the U.S. continue to spiral toward a breaking point, the National Terrorism Advisory System warns of U.S.-based individuals "plotting additional attacks" against pro-Israel targets, and U.S. government and military officials."This is a time for calm, professionalism, information sharing and heightened awareness," Balboni said. "This will not be the first time we face a challenge like this. But certainly because of the hostilities, there is a heightened fear, concern and awareness that these types of attacks could happen in the United States."