After the mass shooting in San Bernardino, Calif., on Dec. 2 that left 14 people dead, details are starting to emerge about the two shooters. Here's what we know about Syed Rizwan Farook and Tashfeen Malik. (The Washington Post)

The California couple who killed 14 people last week at an office holiday party had been adherents of a radical strain of Islam for “some time,” the FBI said Monday as investigators raced to assemble a clearer picture of how the attack was planned.

David Bowdich, assistant director in charge of the FBI’s Los Angeles field office, said investigators have yet to determine whether Syed Rizwan Farook, a 28-year-old county health inspector, and his wife, Tashfeen Malik, 29, had been drawn into violent extremism by someone they knew or whether they had developed those beliefs on their own.

“Both subjects were radicalized and have been for quite some time,” Bowdich said at a news conference here. “How did that happen and by whom and where did that happen? I will tell you right now, we don’t know those answers at this point.”

The FBI was working Monday to confirm reports that Malik, who was born in Pakistan, had ties to Islamabad’s Red Mosque, which is notorious for its connections to Islamic fundamentalism, an FBI official said. Mosque officials have denied any association with her.

Whatever the roots of their beliefs, the couple had prepared carefully for the attack, Bowdich said, visiting local shooting ranges to practice their aim as recently as a few days before the massacre.

David Bowdich, the assistant director in charge of the Los Angeles Field Office, said that the San Bernardino attackers were both radicalized for some time, and participated in target practice within days of the deadly attack on Dec. 2. (Reuters)

The Dec. 2 shooting, which also wounded 21, was the deadliest act of terrorism on U.S. soil since Sept. 11, 2001. The attack has renewed debate over the nation’s gun laws and sparked criticism of President Obama’s campaign against militant groups.

Bowdich said the federal investigation into the shooting is “massive in scale.” So far, authorities have interviewed more than 400 people and collected more than 300 pieces of evidence.

But critical questions remain. Officials have yet to uncover any indication that the attack was plotted with help from overseas, Bowdich said. Nor do they know whether anyone in this country other than Farook and Malik took part in the planning. The couple died in a shootout with police four hours after they opened fire at a conference center in San Bernardino.

On Monday, federal authorities confirmed that Farook’s former neighbor, Enrique Marquez, legally purchased in California the military-grade rifles used in the attack and provided them to Farook. By the time of the attack, the rifles — semiautomatic AR-15s manufactured by DPMS and Smith & Wesson — had been altered for greater lethality.

Marquez, 29, works as a security guard at Walmart and had lived next door to the Farook family for years. The two men shared a love of automobiles, according to neighbors interviewed this week, who said Marquez and Farook could sometimes be seen working on cars together in the neighborhood.

Marquez checked himself into a mental health facility Friday. Authorities said he has since checked out and been questioned by the FBI, which is interested in learning when and why he provided the guns to Farook and whether he had any knowledge of the plot.

Bowdich also clarified that authorities recovered 19 pipes that could be used to assemble homemade bombs during a search of the couple’s home in nearby Redlands, Calif., along with thousands of rounds of ammunition. Authorities had earlier said that 12 pipe bombs were found.

As the probe expands to sites where Malik lived overseas, friends and family in the United States struggled to piece together clues about what may have led the couple to the violence.

The Illinois-born Farook was described as a bright student during his childhood in California. As an adult, those who knew him said, he was a devout Muslim, quiet and private.

Farook brought Malik to the United States on a fiancee visa in July 2014. But friends said they knew little about Farook’s wife. Many weren’t even aware that the couple had welcomed their first child in May 2015, a baby daughter who was placed in the custody of child protective services after the attack.

“At this time I feel like he had a double life,” Saira Khan, Farook’s sister, said in an interview with ABC News. “I feel like he was very good at concealing everything from all of us. The guy that we know, all his co-workers, everybody that knew him at the mosque, they’re all commenting just like we [are]. . . . Nobody knew him any different than how we knew him.”

Much less is known about Malik, who was born in Pakistan but spent at least some time in Saudi Arabia, where her father relocated more than two decades ago. It remains unclear how much time she spent in Saudi Arabia, but she is known to have studied in Pakistan to become a pharmacist.

After arriving in the United States, Malik appeared to have interacted with very few people. Even male relatives said they had never seen her face, which was typically covered by a niqab, or face veil, used by some Muslim women.

Around the time of the attacks, Malik went on Facebook to post a pledge of loyalty to Abu Bakr al-Baghdadi, the leader of the Islamic State. The Islamic State is urging followers to launch attacks wherever they may be as the group seeks to expand its reach beyond Iraq and Syria.

The group has called Syed and Malik followers. But it has not linked itself as clearly to the shooting as it has to a recent series of attacks in Paris.

Five days after the shooting, San Bernardino was slowly returning to normal life. Early Monday, county officials announced that business would resume in most county offices, which had been shuttered since the shooting. The lone exception was the Division of Environmental Health Services, where Farook had worked for years.

Many of the people who died were Farook’s co-workers, which triggered early speculation that the rampage may have been sparked by a workplace dispute. Investigators said that it is not yet clear whether workplace friction might have played some role in the couple’s choice of target.

Since the shooting, San Bernardino has taken steps to enhance security at public facilities, including increasing the number of armed guards. Officials have also established a counseling center and hotline, and managers in county government have been asked to look for signs of stress among their staff.

“The purpose of terrorism is to make ordinary people afraid to do the ordinary things that make up their lives,” said Janice Rutherford, a member of the county Board of Supervisors. “We can’t be afraid of our lives, of our community, of our neighbors, of our coworkers.”

Goldman reported from Washington. Mark Berman and Thomas Gibbons-Neff in Washington contributed to this report.