Revelations that the couple that perpetrated last week's mass shooting in San Bernadino, California, took advantage of K-1 finacé visas have prompted President Barack Obama to order the departments of State and Homeland Security to review the program under which such entry permits are issued.

K-1 visas allow foreign nationals to enter the United States in order to marry U.S. citizens, and almost 36,000 such documents were issued in 2014, Reuters reported based on State Department statistics.

According to the department, seeking such a visa is a multi-step process. The U.S. citizen sponsor must file a petition. The beneficiary must submit an application, a variety of required documents, including police certificates and medical exams, pay a fee and attend an interview at a U.S. diplomatic post abroad.

"It's a very detailed petition," Los Angeles immigration attorney Roman Mosqueda told the Los Angeles Times. "If you're getting married in a church, you have to show proof of getting a church date for the wedding, a date for the restaurant for the reception. ... It's very strict."

Tashfeen Malik, the 29-year-old Pakistan native who came to the U.S. from Saudi Arabia, apparently navigated the K-1 process in order to marry her American-born husband, 28-year-old Syed Rizwan Farook, Reuters detailed. Both individuals were killed last week in a gun battle with police, after they committed the mass shooting in San Bernardino, the Cato Institute recalled.

According to analysis by the libertarian think tank, from 2005 to 2013 Pakistani nationals were awarded a total of 3,177 visas, or 1.2 percent of the approximately 262,162 K-1 visas. Countries whose citizens were among the top beneficiaries of the program, meanwhile, included Canada, China, Colombia, the Dominican Republic, Mexico, the Philippines, Russia, Thailand, the United Kingdom and Vietnam.

"It's not an easy procedure," immigration attorney Paul Herzog summarized, according to the Los Angeles Times. "Beginning to end, you're looking at at least six to nine months" from the date of application to the date of receiving a visa, the lawyer explained.