FBI agents early Saturday raided the home of a man thought 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 deadliest terrorist assault on U.S. soil since Sept. 11, 2001.

As part of its 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.

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 last year, and her husband, Illinois-born Syed Rizwan Farook, pulled together what appeared to be an elaborate attack plan.

Pakistani Interior Minister Chaudhry Nisar Ali Khan told reporters at a news conference on Sunday that his government began their own investigation into Malik last week and uncovered the same biographical details that have been reported in the media. He said the Pakistani government will continue to offer the United States “all possible legal assistance,” but he added that Pakistan should not be held responsible for the actions of a single person.

“No sane Pakistani or Muslim could even think about doing such acts, and only few people are using the name of Islam for their wrongdoings, which is defaming our religion,” he said. “Such heinous acts also lead to serious difficulties for millions of Muslims who live in Western and other countries, and the extremists and nationalist elements in those societies look at Muslims with suspicions. Islamophobia is being spread around the world. What the terrorists are doing has nothing to do with Islam.”

He added that the investigation hadn’t revealed any links between Malik and extremist organizations.

Despite promises of transparency, intelligence agents roamed Pakistani locations linked to Malik in the central town of Multan on Sunday, prompting those who knew her to remain tight-lipped.

The house where she lived until last year was flooded with plainclothes security agents. Beside security officials and a few reporters, the street appeared deserted of normal foot traffic.

Malik had lived there with her mother and sister while attending college, but the family moved in 2014, one neighbor told The Washington Post on the condition of anonymity.

Around the same time, a former college roommate said in an interview, Malik left for Saudi Arabia.

“Tashfeen left Pakistan in August or September 2014, but we lost contact when she got married and moved to the United States of America,” the woman said in a text message, asking that her identity be withheld because of the sensitivity of the matter. “She was initially responding to our calls and Facebook messages but then stopped replying to our messages after December 2014.”

A local cleric who runs a madrassa and mosque near Malik’s former home said that he was aware of the family, but that they largely stayed to themselves.

“I have never seen her father, Gulzar Malik,” said Attaul Manan, the cleric. “The people have been discussing how [Malik] had lived there, but no one ever saw her because the Malik family did not mix with others in the street.”

On Sunday, The Post reported that Malik had also studied at the al-Huda Islamic School in Multan. But it was locked Sunday, and no one was seen nearby.

Three professors at Bahauddin Zakariya University, which Malik also attended, told Reuters that security agencies had told them not to speak to reporters.

A professor, who was not identified, told Reuters security officials had removed records and pictures of Tashfeen from the university on Saturday.

“She was a very reserved person, a very quiet girl, she kept to herself,” he said. “I could have never imagined she was capable of something like this. And there was nothing on the surface to suggest she had such extremist tendencies.”

In the United States, investigators strongly suspect that the couple intended to carry out multiple attacks, according to the news agency Reuters. Investigators lack evidence of other targets but believe that the pair’s huge arsenal — which included a dozen pipe bombs and thousands of rounds of ammunition — suggests future attacks were possible.

During an appearance on Fox News on Sunday, Rep. Michael McCaul (R-Tex.), chairman of the House Homeland Security Committee, said investigators are trying to determine where the couple got the money to buy their weaponry.

“There’s a serious investigation ongoing into what she was doing in Pakistan and in Saudi,” McCaul said. “We think that she had a lot to do with the radicalization process and perhaps with Mr. Farook’s radicalization from within the United States.”

“The wild card here is the wife Malik,” he added.

In his weekly radio address on Saturday, President Obama urged the nation to remain united in the wake of Wednesday’s attack. He urged patience as federal law enforcement continues its investigation.

“We’re working to get a full picture of their motives — why they committed these revolting acts,” Obama said. “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 militant group on Saturday said two supporters had carried out the San Bernardino attack, but according to the Associated Press, it stopped short of saying that the pair were members of the extremist group, which controls parts of Iraq and Syria.

The group has not claimed responsibility for the 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.

Early 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 broken. Two vehicles remained parked in the driveway, and a makeshift sign read: “Please keep off the property.” Neighbors said 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 scouring social media for clues about the attackers’ intentions and are  examining evidence recovered after the attack, including several cellphones that appeared to have been intentionally damaged.

An official who spoke on the condition of anonymity to discuss an ongoing investigation, saidthat  Farook had contact with several individuals in Los Angeles and that they are being tracked by the FBI.

Obama received an update on the investigation on 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 that the couple was “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.”

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.

Three days after the attack, officials were not 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.

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 attack, is in custody of child protective services, at least temporarily.

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