The Dec. 2 San Bernardino, California massacre exposed an open "back door" to the United States through which terrorists can enter easily, authorities revealed.

One of the two shooters, Tashfeen Malik, was a Pakistani who previously resided in Saudi Arabia who was able to come to the United States using a K-1 visa, commonly known as a fiancé visa, when she was petitioned by her husband, Rizwan Farook, who was the other shooter, officials said.

Farook, a U.S. citizen born in Chicago, and Malik met on a dating website and maintained an online relationship prior to Malik coming to the U.S. in July, 2014.

Figures from the U.S. Department of State show the K-1 visa has a much higher admittance rate than practically any other type of visa, the Daily Caller News Foundation found.

"It's frightening because it doesn't take a large leap of the imagination to think that it's possible that ISIS found a back door in using the type of visa that she used," Sen. James Risch, a Republican from Idaho who serves on the Senate Foreign Affairs Committee, told Wolf Blitzer on CNN.

Malik reportedly showed her terrorist nature when she was to have written a Facebook post pledging allegiance to the Islamic State (ISIS) moments after opening fire on her husband's co-workers at the Inland Regional Center in San Bernardino. Her husband Farook also took part in the killing.

The husband-and-wife duo killed 14 people and wounded 21 others before they were shot to death by police in a gun battle.

"You had a man here, who by all accounts, had spent almost three decades here without getting into any trouble — without being radicalised and without visiting sites that are radical sites," Risch said.

"Then, all of a sudden, he hooks up with this woman, who more and more is looking like a black widow, and who comes here.

"And a year and four months later, they have a house full of bombs and ammunition and rifles," he added. "The circumstances are very, very suspicious."

What appears to be even more worrisome was the fact that

Malik, who had been living with her family in both Saudi Arabia and Pakistan, had no criminal record and passed several Department of Homeland Security background checks before entering the United States in July 2014 on a K-1 visa.

That visa allowed her to travel to the country and get married within 90 days of arrival, legalising her presence in the U.S.

U.S. officials said Malik was subjected to a vetting process that was described as vigorous. It included in-person interviews, fingerprints, checks against U.S. terrorists watch lists — as well as reviews of her family members, travel history and places where she lived and worked, WND reported.

This process began when she applied for a visa to move to the United States and marry Farook, a Pakistani-American restaurant health inspector for San Bernardino County who was raised in Southern California.

"It's relatively easy to get a fiancée visa to get into the country," Risch told Blitzer. "That's exactly what happened here. It just strikes me that this thing, the way this happened, it just came together so neatly once they came to America back in July of 2014."

David North, a fellow at The Center for Immigration Studies, warned about the high acceptance rate for fiancé visas years ago, saying that the rate is still climbing with very few cases being rejected.

"It is highly unlikely not to obtain a K-1 visa no matter what the procedure is," North tells The Daily Caller. "The State Department is much more likely to say no to practically any kind of visa that comes before them."