Terrorism experts and former top-ranking law enforcement officials say the United States must not only be prepared to protect against threats caused by the events outside of the country, such as in the Middle East, but must also watch for any threats created by individuals coming in. 

The United States has fully pulled out of Afghanistan after weeks of rushing to evacuate Americans and Afghan allies from the country and its newfound Taliban rule. But several experts, including former top cops in major U.S. cities, warned that the U.S. security measures must be far-ranging, spanning not only from utilizing intelligence and monitoring air travel and airport activity, but also ensuring that those brought into the country have only good intentions.

Former Boston Police Commissioner Ed Davis told Fox News "the great majority of people" seeking refuge from Afghanistan in the U.S. "are looking for a new life – and many of them have helped the United States."

"You don't want to say anything negative about the people who … have actually risked their lives to fight terrorism in a very real way," Davis explained. "But the truth is that when large numbers of people are brought into the country like this, the vetting process has to be very intense. And I know from doing investigations around the world that trying to do a background investigation on someone who has come from a country like Afghanistan is virtually, is very difficult to do." 

Then-Boston Police Commissioner Edward Davis speaks during a media briefing in the parking lot of the Watertown Mall on April 19, 2013 in Watertown, Massachusetts. (Jared Wickerham/Getty Images)

Then-Boston Police Commissioner Edward Davis speaks during a media briefing in the parking lot of the Watertown Mall on April 19, 2013 in Watertown, Massachusetts. (Jared Wickerham/Getty Images) (Photo by Jared Wickerham/Getty Images)

Davis led the Boston Police Department’s response to the 2013 Boston Marathon bombing and served as commissioner from 2006 to 2013. 

Now the president and CEO of private security firm The Edward Davis Company, he told Fox News states like Afghanistan, which he called "a very primitive country," lack databases or systems to record criminal behavior or suspicious activity.

"The federal government has an enormous amount of intelligence that they've gathered by being in the country for 20 years. But it's not perfect," he continued. "And you worry about people slipping through that may have ill intent."

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Ray Kelly, former police and customs commissioner, told Fox News it was important that the U.S. be "very cognizant" of potential risks and "take our time in vetting" those who are coming in. 

World Trade Center bombing Police Commissioner Ray Kelly comes out of the hole with Emergency Service Cops.  (Bill Turnbull/NY Daily News Archive via Getty Images)

World Trade Center bombing Police Commissioner Ray Kelly comes out of the hole with Emergency Service Cops.  (Bill Turnbull/NY Daily News Archive via Getty Images) (Photo by Bill Turnbull/NY Daily News Archive via Getty Images)

"Look, they had to get out of the country in a hurry," Kelly explained. "But now that they're in the U.S., or somewhere in the U.S., I think we have to slow the process way down, and do as thorough background checks as possible."

The Marine Corps veteran was the first person to serve two nonconsecutive terms at the helm of the New York Police Department (NYPD): First, at the time of the World Trade Center bombing in 1993 and again just months after the attacks on Sept. 11, 2001. He also served as commissioner of the U.S. Customs Service, which has since been separated into Immigrations and Customs Enforcement and U.S. Customs and Border Protection. 

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Pace University has previously applauded him for leading the NYPD in its "formidable new capacity to address the threat of terrorism."

Kelly had similar thoughts as Davis regarding the difficulty of vetting individuals coming from Afghanistan.

Taliban fighters wave from the back of a pickup truck, in Kabul, Afghanistan, Monday, Aug. 30, 2021. (AP Photo/Khwaja Tawfiq Sediqi)

Taliban fighters wave from the back of a pickup truck, in Kabul, Afghanistan, Monday, Aug. 30, 2021. (AP Photo/Khwaja Tawfiq Sediqi) (AP Photo/Khwaja Tawfiq Sediqi)

"This is a difficult process, but I still think we have to go through it, because there is definitely a potential threat there," he said. "One person who wants to do us harm and cause an awful lot of damage."

The Taliban is a jihadist militant group that ran Afghanistan in the late 1990s. 

The U.S.-led invasion of Afghanistan in 2001 ousted the insurgents from power, but they never left. After they blitzed across the country in recent weeks, the Western-backed government that has run the country for 20 years collapsed. 

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Dr. M. Zuhdi Jasser, an expert on Islamism, described the ideology of radical Islamist groups as having common elements including "the concept of jihad." He defined jihad as being "a militant conquest to create Islamic states."

"Everyone has a different, some mild variations on that theme. But whether it's ISIS's Islamic State or the Taliban's Islamic State, they're all essentially a theocracy with various forms of coercion built into their autocratic interpretations of their theology," Jasser said.

Jasser, the president and founder of the American Islamic Forum for Democracy, added: "It's almost as if you're looking at … different flavors of soda or whatever it is. The bottom line is, is they're all jihadists, that the biggest threat to their jihad is Western secular democracy and Western principles."

Despite the billions of dollars spent by the U.S. and NATO over nearly 20 years to build up Afghan security forces, the Taliban seized nearly all of Afghanistan in just over a week amid the U.S. troop pullout. The fundamentalist group swept into Kabul on Aug. 15 after the government collapsed and embattled President Ashraf Ghani fled the country.

There are concerns that Afghanistan will once again become a base for militants to plot against the West, much like what lead to the Sept. 11, 2001, attacks that triggered the U.S. invasion. 

Spokespersons for police departments in major cities, including Washington, D.C., Atlanta, Seattle and Los Angeles, have told Fox News they are working closely with federal partners and are continuously monitoring for any potential threats. Representatives of the NYPD and Chicago Police Department did not respond to requests last week seeking information pertaining to any threats and how they are responding. 

Kenn Honig was with the Port Authority of New York and New Jersey for 28 years and, as such, was a first responder to the scene of the 1993 World Trade Center bombing and the attacks on 9/11. During his stint with the Port Authority, he served as the manager of its Emergency Operations Center. He was also a commanding officer for New York’s LaGuardia and John F. Kennedy International airports. 

Honig said that while there is "absolutely no way to 100% guarantee that a dedicated terrorist is not going to attack somewhere," there are certain security measures that he would expect. He identified "visible patrol monitoring," both visible and invisible security, a greater police patrol presence, and monitoring the Internet, including chat rooms and the dark web for any suspicious activity. 

Commissioner Kelly previously told Fox News that departments in major U.S. cities, such as New York, will likely be ramping up security and intelligence efforts to protect against any terror threats posed by the ongoing attacks in Afghanistan.

Speaking about prevention measures that New York City would likely take amid concerns related to Afghanistan, Kelly said investigators are "doing more of what they have been doing."

 (L TO R)  New York Police Commissioner Raymond Kelly, New York Governor George Pataki and New York Mayor Michael Bloomberg bow their heads at a memorial service for those killed during the September 11 attacks at the World Trade Center July 15, 2002, at Fresh Kills Landfill in the New York City borough of Staten Island. (Photo by Spencer Platt/Getty Images)

 (L TO R)  New York Police Commissioner Raymond Kelly, New York Governor George Pataki and New York Mayor Michael Bloomberg bow their heads at a memorial service for those killed during the September 11 attacks at the World Trade Center July 15, 2002, at Fresh Kills Landfill in the New York City borough of Staten Island. (Photo by Spencer Platt/Getty Images) (Photo by Spencer Platt/Getty Images)

For the NYPD, that entails honing in on public sources, working with informants and scouring chatrooms for any signs of suspicious activity, among other methods, he said. The intelligence-gathering skills and resources are already there, Kelly explained, "you’re just ramping up what you’re doing presently."

Former New York Police Department Commissioner Howard Safir told Fox News there was not one single thing police departments in major U.S. cities should be doing. First and foremost, he said, was that law enforcement departments "have to rely on our intelligence agencies to make sure they have enough assets." 

"We need to continue to do what we're doing at airports, and improve it even," said Safir, who served as NYPD police commissioner from 1996 to 1999.  

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He added that he saw the deadly use of airplanes as being "the biggest threat." 

"We have hardened a lot of facilities," said Safir, now the CEO and chairman of a security company called Vigilant Resources International. "We have a lot more people involved."

The Associated Press contributed to this report.