After the mass shooting in San Bernardino, Calif., on Dec. 2, that left 14 people dead, the FBI is unearthing more information about who was involved in the attack. Here's what we know about Syed Rizwan Farook, his wife Tashfeen Malik and his neighbor Enrique Marquez. (The Washington Post)

SAN BERNARDINO, Calif. — Federal authorities believe the Facebook posting from one of the attackers who killed 14 people here last week was made on behalf of both shooters, according to several senior U.S. law enforcement officials.

The Facebook posting said, “We pledge allegiance” to the leader of the Islamic State, using the name Khalifah Abu Bakr al-Baghdadi Al Qurashi, the emir of the self-proclaimed Islamic State. Some of the text in the posting appeared to be garbled, one official said.

This new detail illustrates what federal officials described Monday as the radicalization of both attackers. The FBI investigation continues to explore when the two attackers became adherents to a radical strain of Islam and who may have influenced whom, another law enforcement official said Tuesday.

“It’s looking like they were on the same path at the same time,” that official said.

The FBI remains keenly interested in a former neighbor who provided the military-grade rifles used by Syed Rizwan Farook, the 28-year-old county health inspector, and his wife, Tashfeen Malik, 29, during the massacre that killed 14 people and injured 21 others.

The neighbor, Enrique Marquez, legally purchased the rifles — semiautomatic AR-15s manufactured by DPMS and Smith & Wesson — in California, officials say. The FBI is still investigating whether Marquez sold these rifles to Farook, his former neighbor, according to the officials.

Reports on Monday suggested that Marquez and Farook might have other connections in addition to being neighbors. An official with the Islamic Center of Corona-Norco, in Corona, Calif., the mosque Syed Farook’s brother-in-law Farhan Khan regularly attended, said he believed that Marquez had periodically attended the center. Yousuf Bhaghani, a senior official at the center, said in an interview Tuesday that he had not interacted with Marquez but that other members he had spoken with said they recalled him occasionally praying at the mosque four or five years ago. Those who interacted with Marquez described him as a “decent person,” Bhaghani said. “They didn’t see anything which caused alarm.”

Marquez was also married to a woman who appears to be a relative of Tatiana Farook, the wife of Syed’s brother Raheel, according to county records. Both Marquez and the bride, Russian-born Mariya Chernykh, list the same home address as Raheel Farook, and Raheel and Tatiana Farook were witnesses to the couple’s marriage. The Corona mosque is listed as the couple’s religious center. The family connection was first reported by Reuters.

Marquez, who works as a security guard at Walmart and had lived next door to the Farook family for years, could become a subject of the investigation depending on what the FBI learns, one official said.

Farook and Marquez were friends who 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 about the guns he provided as well as whether he had any knowledge of the plot. The FBI is also investigating whether Marquez and Farook had talked about getting into “badness,” said another of the officials.

An undated combination photo of Tashfeen Malik, left, and Syed Farook. (FBI, left, and California Department of Motor Vehicles via AP)

The FBI says it believes that Malik and Farook, the husband-and-wife killers, were radicalized, but they have not determined yet whether 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,” David Bowdich, assistant director in charge of the FBI’s Los Angeles field office, said at a news conference Monday. “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.”

As part of the investigation, authorities are looking at the financial records of the attackers. The FBI is looking at suspicious financial transactions, including a $28,500 cash loan from an online bank prior to the massacre, an official said.

Fox News first reported that the $28,500 was deposited into Farook’s bank account last month, about two weeks before the shooting.

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. One of the law enforcement officials familiar with the investigation said the FBI was trying to “firm up” her history in Pakistan.

The evidence gathered so far shows that Farook had expressed extreme and radical sentiments well before the shooting, another of the law enforcement officials said. But the official would not say what has been recovered yet from Farook’s computer or other electronic devices, some of which were found smashed into pieces after the attack. Another official said they found evidence that Malik had dabbled online looking at jihadist propaganda.

The FBI was still exploring whether the bombs left behind at the Inland Regional Center after the shooting were meant to target police officers and other first responders in the aftermath of the massacre, a senior U.S. law enforcement official said. These bombs were “potentially viable,” the official said.

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

John Galletta, a shooting instructor the Riverside Magnum Range, said that Farook had come to the range with an AR-15 rifle on the Sunday and Monday before the attacks took place.

Galletta did not personally see Farook, who came alone, but other staff at the range did.

“It was very brief,” Galletta said. “He was there; he shot his gun.”

Staff at the range did not recall having seen Farook before that, Galletta said. Riverside Magnum is small shooting range in a strip mall just off a freeway. The range allows both pistols and rifles, however, rifle shooting — of the likes Farook had — is for members of the range only. ​

Galletta said the range gave authorities the security footage.

“We had no idea he was there until they came,” he said, referring to the FBI.

The Dec. 2 shooting 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.

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 the pledge of loyalty to the leader of the Islamic State, which has urged followers to launch attacks wherever they may be as the militant 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. The county resumed normal business on Monday, reopening offices that 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 and Berman reported from Washington. Thomas Gibbons-Neff, Ellen Nakashima, Julie Tate and Alice Crites in Washington contributed to this report.

Correction: An earlier version of this story incorrectly stated that a mosque Marquez occasionally prayed was also regularly attended by Syed Farook’s brother Raheel. Mosque officials said it was Farook’s brother-in-law who regularly attended the mosque.