Border Force agents check the passports of passengers arriving at Gatwick Airport in London last year. (Oli Scarff/Getty Images)

The recent terrorist attacks in Paris are prompting bipartisan legislative efforts to tighten the security of a program that allows millions of foreigners, mostly from Europe, to travel to the United States without visas.

The visa waiver program, begun in the mid-1980s to spur tourism and business travel, includes 38 countries. Citizens from those nations can enter the United States and remain for as long as 90 days, bypassing the in-person interview at a U.S. consulate abroad traditionally required for a visa, but the travelers are still screened in a variety of ways.

Conceived in a previous era when citizens of traditional European allies were not viewed as threats, the program first raised flags after the Sept. 11, 2001, attacks. Convicted 9/11 plotter Zacarias Moussaoui, a French national, boarded a flight to the United States visa-free, as did foiled “shoe bomber” Richard Reid, a British national. The administration of President George W. Bush responded by tightening security provisions.

But the recent attacks that killed 17 people in France, combined with the influx of foreign fighters into Syria to join the Islamic State and other groups, is triggering heightened concern. With at least 3,000 of those fighters coming from Europe, including an estimated 1,000 from France, some U.S. lawmakers fear that radicalized Europeans could return home, fly to the United States without visas and carry out attacks here.

Sen. Dianne Feinstein (D-Calif.), a leading Senate voice on national security, plans to introduce legislation to strengthen the program’s security. Her office would not provide details.

“There’s no question in my mind that the Visa Waiver Program is a weak link in our efforts to keep violent extremists out of our country and needs to be tightened up,’’ she said in a statement issued to The Washington Post.

In the House, Homeland Security Committee Chairman Michael McCaul (R-Tex.) has voiced similar concerns and recently co-sponsored legislation that would require the Department of Homeland Security — which administers the visa waiver program — to consider additional steps to better screen travelers. McCaul’s committee is investigating the program as part of a broad probe into whether the Obama administration has done enough to stop the flow of foreign fighters.

Few on Capitol Hill are calling for the elimination of the visa-free travel that brings nearly 20 million people to the United States annually — 40 percent of all overseas visitors — amid wide agreement that the visa waiver program facilitates tourism and trade worth millions of dollars to the U.S. economy. And the program also has an unusual bipartisan group of defenders: Not only does the Obama DHS strongly back it, so do top DHS officials from the Bush years.

“I’m not a softie on terrorism, but Senator Feinstein is really missing the security measures we built into the program,’’ said Stewart A. Baker, a former DHS assistant secretary for policy in the Bush administration.

‘Constantly’ evaluated

Baker recently co-wrote a report arguing that visa waivers make Americans more secure, citing requirements that participating countries share information with U.S. authorities about their known or suspected terrorists and criminals, which can then be cross-checked against other databases and used to stop people from traveling to the United States. Proponents of visa waivers also say the traditional visa system is not foolproof, pointing out that the Sept. 11 hijackers entered the United States with visas.

Homeland Security Secretary Jeh Johnson told Feinstein in a letter in December that he is “firmly committed” to the program, and DHS spokeswoman Marsha Catron said the agency is “constantly” evaluating it, including reviewing the potential threat from foreign fighters. Since 2008, she said, U.S. Customs and Border Protection, which is part of DHS, has denied more than 26,000 online applications for visa waivers, some because the applicants were on terrorism watch lists.

The Visa Waiver Pilot Program was established in 1986 as part of comprehensive immigration reform legislation. The goal, proponents recalled, was to stimulate trade and tourism. Most participating countries were European, in part because they were considered to pose less of a security threat than other nations but also because their citizens tended to be wealthier. U.S. officials thought lower-income foreigners would be more likely to exceed the 90-day maximum stay to flee economic difficulties at home.

With the number of countries growing, Congress made the program permanent in 2000. A variety of security measures were added after Sept. 11, especially in 2007, when the law implementing the recommendations of the 9/11 Commission required visa waiver countries to share information with U.S. authorities on their citizens and nationals, including on known and suspected terrorists.

Today, 30 of the 38 visa waiver countries are European, along with a smattering of traditional U.S. allies such as Australia, Japan and South Korea. To travel to the United States visa-free, citizens or nationals of those countries must apply through an online system that requires them to provide biographical information such as their names and passport numbers. If the online system declares them eligible, they can board planes but still must be cleared by customs and immigration when they arrive in the United States. Participating countries allow similar visa-free travel for U.S. citizens.

The process differs from what some on Capitol Hill consider the more rigorous screening for people from other nations, who must submit more information to U.S. officials and, in most cases, be interviewed in person at U.S. consulates before getting visas for travel.

With heightened anxiety across Europe about the threat from radical Islamist groups and the influx of fighters into Syria, “we do indeed believe there is a vulnerability to the United States,’’ said a House Republican aide, who spoke on the condition of anonymity to describe internal deliberations. The aide said the process of granting visas to people from non-waiver nations is “very thorough,’’ in contrast to the visa waiver program, which collects only a limited amount of information about travelers.

Feinstein agreed, saying that unless a traveler is already known or suspected to be a terrorist, “the process to secure a visa waiver and travel to the United States is relatively simple.’’

The Obama administration has acknowledged the potential threat from Islamic State-trained fighters going home to Europe and then targeting the United States. “Of particular concern are Western passport holders,’’ Lisa Monaco, assistant to the president for homeland security and counterterrorism, said in a September briefing for reporters.

Citing in particular French and British citizens, she said they constitute “a very potent mixture, and pose a potential threat, if left unchecked, to the homeland.”

Mitigating the danger

But current and former DHS officials, while not denying a potential threat, said the security measures imposed starting in 2007 have substantially reduced the threat while maintaining the program’s economic benefits.

Catron, the DHS spokeswoman, said the visa waiver program’s online application — known as the Electronic System for Travel Authorization — “has been an important security and vetting tool” that has allowed DHS to deny travel to some applicants, including more than 22,000 who used lost or stolen passports.

In November, DHS created additional data fields in the system, including more passport information and other names or aliases the travelers may have used. The agency is considering additional security enhancements.

Under the program, DHS is also sent electronic passenger manifests from airlines and commercial vessels traveling to the United States. Customs inspectors check them against a variety of security databases and the information provided by the travelers’ home countries, a process described as thorough by current and former officials.

Nathan A. Sales, a Syracuse University law professor and senior DHS official in the Bush administration, said he thinks Feinstein is “100 percent right about the threat. Absolutely we face and have faced for years a very significant threat from radicalized populations in Western Europe.”

“But the solution is not to kill the program, and the solution is not to go back to the old visa-based system of screening passengers,’’ he said. “The solution is to strengthen the security features of the program.’’