WASHINGTON: The woman who carried out the San Bernardino massacre with her husband came to the United States (US) last year on a special visa for fiancé's of US citizens, raising questions about whether the process can adequately vet people who may sympathise with terrorist groups.

Authorities said Friday that Pakistani citizen Tashfeen Malik, 27, pledged allegiance to the militant Islamic State (IS) group and its leader under an alias account on Facebook just moments before she and her husband, Syed Farook, opened fire on a holiday banquet for his co-workers Wednesday, killing 14.

They later died in a gun battle with police.

Read: Suspects Syed Farook, Tashfeen Malik kill 14 in California shooting: authorities

Malik, who had been living with her family in Saudi Arabia and Pakistan, passed several government background checks and entered the US in July 2014 on a K-1 visa, which allowed her to travel to the US and get married within 90 days of arrival.

Malik was subjected to a vetting process the US government describes as vigorous, including in-person interviews, fingerprints, checks against US terrorist watch lists and reviews of her family members, travel history and places where she lived and worked.

The process began when she applied for a visa to move to the US and marry Farook, a 28-year-old Pakistani-American restaurant health inspector who was raised in California.

Foreigners applying from countries recognised as home to Islamic extremists, such as Pakistan, undergo additional scrutiny before the State Department and Homeland Security Department approve permission for a K-1 visa.

"This is not a visa that someone would use because it is easy to get into the US, because there are more background checks on this type of visa than just about anything else," said Palma Yanni, a Washington-based attorney who has processed dozens of K-1 visas. "But fingerprints and biometrics and names aren't going to tell you what is in somebody's head unless they somewhere have taken some action."

The government's apparent failure to detect Malik's alleged sympathies before the shootings will likely have implications on the debate over the Obama administration's plans to accept Syrian refugees.

Attorneys representing Farook's family deny that he or his wife had extremist views.

On Friday, ABC News reported that the address in her Pakistani hometown that Malik listed on her visa application does not exist.

In response to a question about the ABC report, State Department spokesman Mark Toner said: "We are actively reviewing all of the information provided in the visa application and sharing it with our inter-agency partners as it relates to the investigation."

The vetting process for refugees is similar, though not identical, to the one for fiancé-visa applicants.

Refugees submit to in-person interviews overseas, where they provide biographical details about themselves, including their families, friendships, social or political activities, employment, phone numbers and email accounts.

They provide biometric information, including fingerprints. Syrians are subject to additional classified controls.

Those who come to the US on a fiancé visa must marry a US citizen within 90 days or leave the country.

Following the marriage, the immigrant becomes a conditional resident for two years and must ask the US government to remove those conditions at the end of that waiting period and undergo another background check.

If the request is approved, the immigrant receives a green card. Immigrants can apply to become US citizens five years after winning a green card.

"Can we improve the system as technology grows? There is always room for improvement, but to indict the entire fiancé visa system because of this is not the right path," said David Leopold, a past president of the American Immigration Lawyers Association.

Those who intersected with Malik in California could not offer much insight, as she was rarely seen in the Muslim community.

The couple was married August 16, 2014, and held their wedding reception at the Islamic centre of Riverside, said Dr. Mustafa Kuko, the centre's director.

Kuko said he never met Malik because the party was divided into separate spaces for women and men.

"She never came to our mosque except once when they had their reception, and that night there were so many people around, my wife doesn't recall exactly how she looks or who she is," Kuko said. “We never saw her again.”