"100% security is very difficult to achieve 100% of the time," Kirby told reporters, though he insisted the screening process is "very rigorous" and "thorough."
"If somebody had mal intent going into that [process], and they had no prior record that would lead anybody in law enforcement or the security establishment to question their motives, you could easily see where somebody could dupe the system," Kirby said.
Kirby spoke broadly about the K-1 visa process by which applicants with an American fiance or fiancee seek entrance into the United States, but he would not comment specifically on the San Bernardino case, which is currently under investigation.
"The process is handled collaboratively between [the Department of Homeland Security] and the State Department," said Kirby. "Obviously, the State Department's role is primarily overseas with the fiance that is applying for the K-1 visa program."
"It's a very rigorous, thorough screening process," he added. "It is also case by case."
What does that process entail?
Applicants for K-1 visas must submit an application, get fingerprinted, undergo an interagency background check and submit to an in-person interview with a consular officer.
The process includes an interagency security vetting procedure during which applicants' biographical and biometric information is cross-checked with databases at the State Department, Department of Homeland Security, FBI and the National Counterterrorism Center, according to a State Department official.
Applicants from certain "high-threat" embassies and consulates undergo additional vetting by Immigration and Customs Enforcement (ICE) and Customs and Border Protection personnel, who manually check their information against information at the Department of Homeland Security.
Consular officers take into account the local conditions in the country where the applicant is from when conducting the interview, the official said.
The security process for K-1 applicants matches the process other visa applicants undergo, although K-1 applicants also have to demonstrate they are in a real relationship with their American fiance, sometimes by answering questions about the person.
What questions do applicants have to answer?
The application paperwork includes questions about terrorist activity, such as, "Do you seek to engage in terrorist activity while in the United States, or have you ever engaged in terrorist activity?" and "Are you a member or representative of a terrorist organization?"
"It would be, obviously, imprudent for us not to ask these questions," said Kirby, "but we're not foolish enough to [not] also realize that even those who might actually have these proclivities would answer 'no' to those questions."
A senior State Department official told CNN on Wednesday that Malik was not asked about jihadist leanings during her interview in Pakistan last year because no red flags were raised in her application
And as presidential candidate Donald Trump faces controversy for his proposal to stop all Muslims from entering the
United States, it's important to note that visa applicants are not asked about their religious beliefs. In fact, U.S. immigration law does not allow the State Department to reject an applicant based on religion.
Can anything be done to limit the risk?
President Barack Obama has called for a review of K-1 visa adjudications in light of the San Bernardino shooting, and that review is underway.
"We're going to vigorously work through this review process," Kirby said, adding, "if we find something about the process that needs to be fixed, we'll fix it. "
But, Kirby said, no system can be "100% effective."
There are no caps on the number of K-1 visas the State Department issues each year. In the 2014 fiscal year, 35,925 K-1 visas were issued worldwide, according to State Department data