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)

SAN BERNARDINO, Calif. — Authorities now believe that the two attackers who killed 14 people here last week had been radicalized for quite a while, an FBI official said Monday.

“Both subjects were radicalized and have been for quite some time,” David Bowdich, the assistant director in charge of the Los Angeles Field Office, said at a news conference.

Bowdich touched on the question of whether one of them guided the other down the path of radicalization, saying that police did not know yet when or how this might have occurred.

“How did that happen and by whom and where did that happen?” he said. “I will tell you right now, we don’t know those answers at this point.”

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

He added that it was possible there was no “whom” responsible for this, but said investigators were still exploring all possibilities.

Investigators have found evidence of “pre-planning,” Bowdich said. Both attackers went to shooting ranges in the Los Angeles region for target practice, including one such visit just days before the shooting, he said.

The federal investigation — which Bowdich described as “massive in scale” — has involved more than 400 interviews so far, he said.

San Bernardino County officials had said earlier Monday they were returning to work in an attempt to resume normal business, five days after the massacre at an event for government workers.

The only exception is the Division of Environmental Health Services, where work will be suspended for at least another week, county officials said at a news conference. One of the two killers, Syed Rizwan Farook, was a department employee.

In an effort to help employees cope with the effects of last week’s violence, the county has established a counseling center and hotline, and managers in county government have been asked to look for signs of stress among their staff.

A photo of Tashfeen Malik, center, and Syed Rizwan Farook, right, going through Chicago’s O’Hare International Airport in 2014. (ABC News)

San Bernardino has already taken steps since the Dec. 2 shooting at the Inland Regional Center to enhance security in county facilities, including increasing the number of armed guards at certain facilities, and is considering additional measures.

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County officials and physicians from hospitals that treated shooting victims stressed that San Bernardino would seek to remain united and would rally around those affected by the massacre.

“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.”

Of six victims who were admitted to Arrowhead Regional Medical Center after the attack, five have gone home.

The wide-ranging investigation into the deadliest terrorist attack on U.S. soil since Sept. 11, 2001, continued Monday, as officials sought answers about the attack, the suspects and any ties they may have to other people.

Law enforcement agents on Sunday again searched the home of a man suspected of providing Farook with the military-grade rifles he and his wife used to gun down 14 people.

A team of three officials spent about 30 minutes inside the home of Enrique Marquez, a former neighbor of Farook’s, and left carrying a large cardboard box. The box’s contents were unknown, and the officials declined to identify themselves or their agency to reporters outside.

The search at the modest suburban home, the second in two days, took place as new details surfaced about Marquez, who officials say bought the DPMS and Smith & Wesson AR-15 rifles that Farook and his wife, Tashfeen Malik, used in the Dec. 2 rampage.

Marquez, who works as a Walmart security guard, checked himself into a mental health facility Friday; it is not yet clear whether he has already been questioned by authorities or if he will be charged.

On Monday, in response to reports that Marquez was detained or charged, Bowdich said he would not comment on Marquez’s status.

A picture has begun to take shape of the couple’s past and their apparent radicalization even as their lives followed a seemingly ordinary course: work, marriage, child. The couple’s daughter, born in May, is in the care of child protective services.

A makeshift memorial site honoring shooting victims. (Jae C. Hong/AP)

The Marquez and Farook families lived next door to each other on the quiet residential street in Riverside, Calif., for years. Like Farook, Marquez, 29, attended La Sierra High School. Both young men shared a love of cars.

Neighbor Jared Rork said Farook and Marquez would sometimes be seen working on cars and listening to music in one of their garages. “They seemed like everyday, normal Americans,” he said.

One of his coworkers at Walmart, who asked to go only by his last name, Dandy, said Marquez had worked at the store only a few months.

When he heard about Marquez’s possible role in supplying the weapons used on Dec. 2, Dandy was shocked.

“I was just blown [away], man,” he said. “I couldn’t believe something like that could come here.”

Friends and family described Farook, who was born in Illinois and grew up in California, as devout and conservative. Those who prayed with him at local mosques said the 28-year-old spoke about his personal life to few people.

On Sunday, Italian publication La Stampa published an interview with Farook’s father, also named Syed, in which he said his son had harbored anti-Semitic animosity. Reached at his son Raheel’s home on Sunday morning, the elder Farook said his views differed from those of his son.

“He was going towards [conservatism],” he told reporters through the gate of the home. “His views were conservative, my views were liberal.”

He also said that Syed Farook had quarreled with a Jewish coworker. One of the fellow county health inspectors killed in the attack was Nicholas Thalasinos, who converted to the Messianic Jewish movement of Christianity three years ago and who frequently posted online about Israel and politics.

Kuuleme Stephens, a friend of Thalasinos’s, said she had spoken with him by phone about two weeks before the attack, reaching him when he was having a conversation with Farook about the nature of Islam and Israel’s place in the Middle East.

But she played down the significance of the conversation. “It wasn’t an argument,” she said. “No one was raising their voices.”

Investigators are also working to determine what other attacks the couple may have planned. The rifles they used had been altered to make them more lethal, and a major arsenal was found in their two-story townhome, including what authorities said were a dozen pipe bombs and thousands of rounds of ammunition.

On Monday, Bowdich said authorities had found 19 pipes that could have been used for pipe bombs, rather than the dozen pipe bombs previously described.

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 establish 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.

[From pharmacy student to suspected San Bernardino terrorist: The baffling journey of Tashfeen Malik]

One of the few known clues about Malik’s beliefs was a posting she made on Facebook around the time of the attack, pledging loyalty to Islamic State leader Abu Bakr al-Baghdadi.

The Islamic State has described the couple as followers but has not expressed the same close association with the attack that it has with other recent examples of violence, such as the Nov. 13 attacks in Paris.

On Sunday, Pakistani Interior Minister Chaudhry Nisar Ali Khan told reporters that his government was investigating Malik’s past.

He said Pakistan was offering the United States “all possible legal assistance” but added that Pakistan should not be held responsible for the actions of a single person.

“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,” he said.

So far, the Pakistani government has not found any links between the 29-year-old Malik, who was born in Pakistan, and extremist groups, he said.

Reporters’ ability to look into Malik’s roots was made more difficult by the presence of intelligence agents at sites she is known to have visited in the town of Multan, where she lived with her mother and sister while undertaking her college studies. The family moved in 2014, one neighbor said.

On Sunday, 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 that security officials had removed records and pictures of Malik from the university Saturday.

A former college roommate, speaking on condition of anonymity because of the sensitivity of the matter, said she stayed in touch with Malik after she moved to the United States in the summer of 2014, but gradually lost touch.

“She was initially responding to our calls and Facebook messages,” the woman said in a text message. “But then [she] stopped replying to our messages after December 2014.”

A local cleric, Attaul Manan, who runs a madrassa and mosque near Malik’s former home, said the Malik family largely stayed to themselves. “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,” he said.

But even the most basic facts of Malik’s background remain disputed.

While Pakistani officials say she lived in Saudi Arabia on and off for 25 years, the Saudi government on Sunday said she had spent little time there.

According to the Associated Press, Saudi Interior Ministry spokesman Maj. Gen. Mansour al-Turki said she had spent less than six months there in two visits in 2008 and 2013.

Craig reported from Pakistan. Abby Phillip and Thomas Gibbons-Neff in San Bernardino, Calif., and Alice Crites, Karen DeYoung, Adam Goldman and Peter Holley in Washington contributed to this report.