Federal investigators believe there is a "very serious" possibility that Tashfeen Malik, one of two shooters who murdered 14 people and wounded 21 others in San Bernardino, Calif. Wednesday, radicalized her husband and co-assailant, county restaurant inspector Syed Farook, Fox News has learned. 

Investigators also believe that the couple had planned a second attack after the shooting at a social service center for the disabled when they were killed in a shootout with local authorities approximately two miles away.

Little is known about Malik's background prior to her meeting Farook. However, a U.S. official confirmed to Fox News that the two met and became engaged after Farook traveled to Saudi Arabia in September 2013. Malik, a Pakistani citizen, applied for a K-1 visa at the American embassy in Islamabad in May 2014 and Farook traveled to Saudi Arabia that July to bring her to the U.S. The Saudi Embassy in Washington has confirmed that Farook's 2014 trip lasted nine days.

They were married on Aug. 16, 2014, in nearby Riverside County, Calif. according to their marriage license. Both listed their religion as Muslim.

Investigators believe that on at least one of those trips to Saudi Arabia, one or both members of the couple made contact with suspected Al Qaeda terrorists. The exact nature of that contact was not immediately clear.

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Rita Katz, director of SITE Intelligence Group, an organization that tracks and analyzes extremists, told the Associated Press it hasn't found any connection between Farook and jihadi groups. But she also said that some of Farook's social media posts seem to have been deleted before the attack.

Law enforcement sources told Fox News late Thursday that there was a "very strong" possibility that Malik functioned as Farook's terror trainer and may have even put together pipe bombs found by authorities at the various crime scenes Wednesday. 

Officials said Thursday that Malik underwent and passed a Department of Homeland Security counterterrorism screening as part of the process of getting the K-1 visa. The visa would have been effective for 90 days, after which Malik would have had to apply for green card status through the Department of Homeland Security as the wife of an American. It was not immediately clear whether she did so.

Wearing black tactical gear and wielding assault rifles, Farook, 28, and Malik, 27, sprayed as many as 75 rounds into a room at the Inland Regional Center, where about 75 of Farook's co-workers had gathered Wednesday morning. Farook had attended the start of the event but slipped out and returned in battle dress.

Four hours later and two miles away, the couple died in a furious gun battle in which they fired 76 rounds, while 23 law officers unleashed about 380, police said.

As part of the complex investigation late Thursday, authorities were trying to piece together a money trail that would have enabled the suspects to acquire over $30,000 worth of guns and explosives. Public records show that Farook made approximately $51,000 per year as an employee of the San Bernardino County Department of Public Health, making it unlikely he could have afforded such an arsenal out of his own pocket. There is no evidence that Malik had a job.

Among the weapons found were three rigged-together pipe bombs at the social service center, each equipped with a remote-control detonating device that apparently malfunctioned; more than 1,600 rounds of ammunition and multiple pipe bombs in the rented SUV where they died; and 12 pipe bombs, tools for making more, and over 3,000 additional rounds of ammunition at a family home in the nearby town of Redlands. 

Officials were also looking for a man who bought the two AR-15 rifles the couple used in the attack. The Associated Press reported that Farook legally bought two pistols found on the couple, but an unidentified man bought the rifles. It was not immediately clear whether the man was acting on Farook's behalf when he bought the weapons or if they were stolen and then used in the attack. A law enforcement official told the Associated Press that all the guns were purchased legally in California. 

Law enforcement sources told Fox News that investigators believe the couple's death prevented a second attack Wednesday, though they have not established what the target would have been. They point not only to the sheer amount of weaponry left unused, but also to the fact that the couple's rented SUV was due to be returned Thursday. Authorities believe it to be unlikely that the couple would have risked the SUV and any unused munitions being found if the vehicle was kept longer than its rental period. 

Farook was a devout Muslim who prayed every day and recently memorized the Koran, according to brothers Nizaam and Rahemaan Ali, who attended Dar Al Uloom Al Islamiyah mosque in San Bernardino with Farook.

Rahemaan Ali said he last saw Farook three weeks ago, when he abruptly stopped going to the mosque. Ali said Farook seemed happy and his usual self, and the brothers never saw a violent side.

"He never ever talked about killing people or discussed politics, or said that he had problems at work," Rahemaan Ali said. "He always had a smile on his face."

A profile on a matchmaking website for South Asians that matched Farook's name, California hometown, county health job and Muslim faith said his interests included target shooting in his backyard. Though the date of the posting was not clear, it listed his age as 22, so it could have been six years old.

Two weeks ago, Farook and one of the co-workers he killed, 52-year-old Nicholas Thalasinos, had a heated conversation about Islam, according to Kuuleme Stephens, a friend of the victim.

Stephens said she happened to call Thalasinos while he was talking with Farook at work. She said Thalasinos told her Farook "doesn't agree that Islam is not a peaceful religion."

Fox News' Adam Housley and the Associated Press contributed to this report.