The threat of a major terror event taking place on U.S. soil is at a dangerously high level with big cities like New York City and Washington, D.C. most vulnerable to an attack, terror experts are warning.

It's been over 23 years since nearly 3,000 people perished on 9/11, but even though surveillance and airport security has been beefed up, the world has changed considerably since then.

Technology has become cheaper and more advanced, the internet is lightning fast and America’s open border means millions of unvetted people from all over the world are now living among the general population and continue to pour in at record rates.

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"We are in a period where a number of factors are combining to make a terrorist attack on the homeland far more likely than it perhaps had been," says Paul Mauro, a retired NYPD inspector who worked in counterterrorism for nearly 15 years.

Mauro said the risk of a potential new attack should be assessed in a criminal context – who has the means, motive and opportunity to successfully execute a deadly plot. 

"The world has gotten a lot smaller since 9/11. You've got drones, the interconnectivity of the internet is more ubiquitous into areas that maybe didn't have that connectivity. You can get an uplink using something like an Elon Musk satellite. You can get online from the mountains of the Hindu Kush, you've got AI capabilities, you've had all that time to develop capabilities since 9/11 if you are still in the [terror] business."

Mauro said that threats would likely emanate from current hot spots around the globe, including the Middle East, where Iran is a live threat and is still seeking revenge for the killing of Qasem Soleimani, an Iranian major general who was taken out in a 2020 U.S. drone strike.

Those angered by Israel’s response to the Oct. 7 attacks in Gaza also pose a real threat, as does a "nuclear capable" Russia, which has been battling against a U.S.-funded regime in Ukraine, Mauro said. 

"It's not hard to see motives all over the place, motives that preexisted 9/11 and that continue and have only been exacerbated," he said.

Mauro said the opportunity for terrorists lies at the southern border.

"We’ve let 10 to 20 million illegals into the country with little or no vetting. It's a canard to think that they're all just from small towns in Mexico or Bolivia or Ecuador or something," Mauro says. 

"We're seeing a wave of migrant-related crime developing now, but you would have to be a fool not to take advantage of the ability to insert assets seamlessly and with no friction into the American homeland through the southern border… if you're sitting where Iran is, where China is, where Russia is."

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Mauro said that the eight Tajikistan nationals with ties to ISIS who were recently nabbed having allegedly crossed the southern border illegally was "an enormous red flag" and that there are millions of unvetted people streaming across the border in the post-9/11 world is "just utterly incomprehensible."

Former CIA Deputy Director Mike Morrell also sounded off in a recent Foreign Affairs op-ed, writing that the terror threat posed by vulnerabilities at the U.S.-Mexico border is "blinking red."

FBI Director Christopher Wray and U.S. Central Command (CENTCOM) Gen. Michael Kurilla said there are serious threats posed by terrorist groups like al-Qaeda, ISIS, also known as the Islamic State and Islamic State Khorasan (ISIS-K), the ISIS affiliate that operates in Afghanistan and Pakistan.

Forces under Kurilla’s command have been battling ISIS targets in Iraq and Syria, killing or capturing almost 1,000 terrorist fighters last year in nearly 500 ground operations and 45 airstrikes, according to Foreign Affairs. 

Kurilla, in March 2023 testimony to the Senate Armed Services Committee, called particular attention to ISIS-K and warned that the group would be able to carry out an operation against U.S. or Western interests abroad in under six months with little to no warning. In January, two ISIS-K suicide bombers killed nearly 100 people and wounded scores at a ceremony in Iran to commemorate Soleimani’s death, while in March the group killed 145 people and injured 550 in a terrorist attack at a Moscow concert hall.

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Wray, meanwhile, told a Senate Appropriations subcommittee earlier this month that there is an increasing concern about an attack similar to the Moscow concert hall and that threats from foreign terrorists have risen to another level.

"Just in the time that I’ve been FBI director, we’ve disrupted multiple terrorist attacks and cities and communities around the country," Wray said, while making his case on the department’s request of $11.3 billion for FY25, or $661 million more than last year.

Wray went on to say that the FBI has seen a "rogues' gallery" of foreign terrorist organizations call for attacks on the U.S. and its allies.

"Given those calls for action, our most immediate concern has been that individuals or small groups will draw a twisted inspiration from the events in the Middle East to carry out attacks here at home," Wray said. "But now, on top of that, increasingly concerning is the potential for a coordinated attack here in the homeland, not unlike the ISIS-K attack we saw at the Russian concert hall back in March."

Attorney General Merrick Garland also sounded the alarm earlier this month on potential terror threats, warning before the House Judiciary Committee that the threat level "has gone up enormously" since Oct. 7.

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Mauro said that New York City and Washington, D.C. remain the top two areas where terrorists are likely to strike, with several other densely packed cities also next on the list. 

In terms of what type of terror attack we could see, it is anyone’s guess, Mauro said. 

"The Holy Grail, I will tell you, and I'm not giving anything up by this… is a bio-chem event for the deployment of that weaponry. You get the most potential loss of life. You get contamination in an enclosed environment... and it's achievable, whereas the dirty bomb scenario which you often hear about, there are scientific and technical hurdles to overcome there that make that difficult," Mauro says. 

"Never mind a real atomic bomb that you could get. Let's say Hezbollah buys it on the black market from some former KGB officer or something, sails it into a harbor and detonates… it was always the nightmare scenario."

"But what punches at a higher weight is the kind of attempts that we've seen around the world employing biochem agents and [it] really is the nightmare scenario. I mean, look at the damage that COVID did."

"So if you could manage to set off that kind of a thing in an enclosed area, in a train station, on a train, etc., you've got a serious, serious event. And that's one of the things that the kind of terrorism folks that I used to be a part of worry about all the time and work to prevent."

Fox News’ Greg Wehner contributed to this report. 

California Gov. Gavin Newsom, a Democrat, announced Tuesday his pledge to restrict students' smartphone use during the school day, pointing to statements from the Biden administration that social media harms the mental health of children.

The decision comes after U.S. Surgeon General Vivek Murthy urged Congress to require warning labels on social media platforms about their impact on young people, similar to the warning labels on cigarettes and alcohol about their impacts on a person's health, according to POLITICO, which first reported Newsom's announcement.

Newsom and his wife, Jennifer Siebel Newsom, have warned for years about the harm of social media to children, saying tech companies have been blocking efforts to protect young people.

Last year, Newsom called on NetChoice to drop a lawsuit against the children's online safety law, the Age-Appropriate Design Code Act, that he signed in 2022. NetChoice, whose members include tech giants like Meta, Amazon and Google, is a trade association that advocates for free expression and free enterprise on the internet.

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The governor said Tuesday he plans to build on a law he signed in 2019 that gave school districts the power to limit or ban students' use of smartphones during school hours. He said he would work with his Democratic-controlled Legislature during the current session to pass a measure to restrict smartphone use in schools.

"As the Surgeon General affirmed, social media is harming the mental health of our youth," Newsom said in a statement. "Building on legislation I signed in 2019, I look forward to working with the Legislature to restrict the use of smartphones during the school day. When children and teens are in school, they should be focused on their studies — not their screens."

The California School Boards Association said school districts should be making decisions on whether to regulate smartphone use in schools, rather than the state.

"We support legislation which empowers school leaders to make policy decisions at a local level that reflect their community's concerns and what's necessary to support their students," California School Boards Association spokesperson Troy Flint told The Associated Press.

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The Los Angeles Unified School District board voted Tuesday for the district to establish policies prohibiting students' smartphone use during the school day, with some exceptions. Board Member Nick Melvoin noted how "students are glued to their cell phones, not unlike adults."

"When I talk to teachers and students and parents and principals, I also hear the same, which is that more and more time is being spent on policing student phone use," he said at the meeting. "There's not coherent enforcement, and they're looking for some support from the board and from the district."

Newsom's announcement is noteworthy as California's Silicon Valley is where many tech companies are located.

The decision also puts the California governor on the same side of the debate as Florida Republican Gov. Ron DeSantis, who earlier this year signed one of the most restrictive bans in the country on children's social media use.

In recent years, a California proposal to fine social media platforms for addicting children has failed to become law. However, Democrat state Sen. Nancy Skinner's bill to ban platforms from providing addictive feeds to children passed the state Senate in May.

"A warning label is important, but we also need to provide parents with tools to protect their kids from preventable harms," Skinner told POLITICO, saying her bill would complement Murthy's proposal.

State Sen. Henry Stern, a Democrat, introduced a bill earlier this year to expand school districts' power to restrict social media use for students during school. He said he would be open to pulling his bill, which already passed the Senate, if Newsom and the Legislature can find a better alternative.

"It's just too hard for every teacher, every school, or every parent to have to figure this out on their own," Stern told The Associated Press. "There's some times where government just has to step in and make some bigger rules of the road."

The Associated Press contributed to this report.