SAN BERNARDINO, Calif. - FBI agents early Saturday raided the home of a man believed to have purchased two military-grade rifles used in last week's deadly rampage in San Bernardino, Calif., as federal authorities raced to piece together clues about the most deadly terrorist assault on U.S. soil since Sept. 11, 2001. 

As part of their expanding investigation, the FBI is now working to determine whether the man, a former neighbor of one of the attackers, had any knowledge of the plot, which killed 14 people and wounded 21 at a holiday party for county health inspectors.

President Obama is expected to discuss the San Bernardino investigation and other terrorist threats from groups such as the Islamic State in an address from the Oval Office on Sunday evening.

Investigators are pursuing leads as far away as Pakistan and Saudi Arabia that they hope will explain how Tashfeen Malik, a Pakistani woman who immigrated to the United States in 2014, and her husband, Illinois-born Syed Rizwan Farook, pulled together what appeared to be an elaborate attack plan.

"We're working to get a full picture of their motives - why they committed these revolting acts," Obama said in a weekly address Saturday. "We're going to get to the bottom of this."

The incomplete picture of the attackers and their motives reflects the difficulty of detecting and preventing attacks by individuals with few or no substantial connections to militant organizations overseas.

An audio message broadcast by the Islamic State on Saturday said that two supporters had executed the San Bernardino attack, but according to the Associated Press it stopped short of saying the pair were members of the extremist group, which controls parts of Iraq and Syria.

The group has not claimed responsibility for Wednesday's bloodshed, as it did for other recent attacks, including the coordinated assaults that killed 130 people in Paris.

While officials said they have not uncovered evidence to suggest that Farook and Malik were part of a larger militant organization, Malik appears to have admired the Islamic State. Around the time of Wednesday's attack, Malik, who gave birth to the couple's first child in May, pledged allegiance on Facebook to the group's leader, Abu Bakr al-Baghdadi.

The Islamic State, under pressure after over a year of U.S. and allied airstrikes, has urged its followers to launch attacks in the West.

U.S. authorities are now seeking to determine how the couple, who were living with Farook's mother in a modest two-story townhouse in Redlands, a city close to San Bernardino, prepared for the attack. Investigators discovered a huge arsenal, including a dozen pipe bombs and thousands of rounds of ammunition.

Early on Saturday, law enforcement authorities raided a home next door to one where the Farook family once lived in Riverside, Calif. According to a law enforcement official, the raid targeted Enrique Marquez, who is thought to have bought the two military-grade rifles used in the attack. Both weapons were modified in a way that allowed them to be used with greater lethality, suggesting extensive planning for the attack.

Marquez, who has not been charged with a crime, has checked himself into a nearby mental health facility.

At the site of the raid, the garage had been partially torn down and a window was broken. Two vehicles remained parked in the driveway, and a makeshift sign read: "please keep off the property." Neighbors said that Marquez, who lived in the quiet neighborhood for more than a decade, shared a hobby with Farook: cars.

"They had a common interest in mechanics and cars, said Freddy Escamilla, 21, who has lived across the street for most of his life.

"They would walk over to each other's yards to talk to each other," said Escamilla, who added that he saw the two working on cars together. "It never seemed out of the ordinary."

Law enforcement officials are now scouring social media for clues about the attackers' intentions and are also examining evidence recovered after the attack, including several mobile phones that appeared to have been intentionally damaged to prevent their exploitation.

An official who spoke on the condition of anonymity to discuss an ongoing investigation, said Farook had contact with several individuals in Los Angeles who were already being tracked by the FBI.

Obama received an update on the investigation Saturday morning from FBI Director James B. Comey, along with Attorney General Loretta E. Lynch and Homeland Security Secretary Jeh Johnson, the White House said in a statement.

Information that has emerged since the attack suggests the couple were "radicalized to violence to commit these heinous attacks," the statement said.

In his weekly address, Obama said the attacks, if motivated by radicalization, "would underscore a threat we've been focused on for years - the danger of people succumbing to violent extremist ideologies."

Three days after the attack, officials were not yet certain whether Malik or Farook was a primary instigator of the attack, which has triggered fears from American Muslims about being characterized as potential militants.

But a more clear picture is emerging of Malik, who was born to a prosperous family in Pakistan and studied pharmacology at a university there. Those who knew her said she grew more conservative over time.

After her arrival in the United States in the summer of 2014, Malik avoided being in the room with her new male relatives. Because she used a face covering used by some Muslims, even her brother-in-law had never seen her face.

Malik also spent years living in Saudi Arabia. Officials there have said she was not on any security watch list.

Lawyers representing relatives of Farook, including his mother, three siblings and brother-in-law, have described Farook and Malik as an average, if isolated, couple. Malik is known to have been seen outside the couple's home only a handful of times.

Acquaintances from local mosques said that Farook was devout, quiet and intensely private about his personal life. He is said to have stopped attending his mosque in San Bernardino months ago.

Law enforcement officials now think that Farook - who, like many of those killed, worked as a county health inspector - was the among the first to arrive at Wednesday's gathering at the Inland Regional Center in San Bernardino. That suggests he may have been scoping out the facility. It also could tamp down speculation that the attack was a more spontaneous workplace dispute triggered by an argument.

The couple's infant daughter, who was born at a local hospital and was left with Farook's mother while Malik and Farook carried out the attacks, is in custody of child protective services, at least temporarily.

The Washington Post's Goldman reported from Washington.Thomas Gibbons-Neff and William Dauber in San Bernardino, California; Tim Craig in Islamabad, Pakistan; and Julie Tate, Karen DeYoung, Ellen Nakashima and Alice Crites in Washington contributed to this report.