Investigators believe the married couple who massacred 14 people in California last week - a US-born husband and his Pakistani wife - had been radicalised "for quite some time," but no clues pointing to an international plot have yet emerged, the FBI said on Monday.

Authorities also have evidence that Syed Rizwan Farook, 28, and his spouse Tashfeen Malik, 29, had engaged in firearms target practice near their Southern California home within days of last week's deadly shooting rampage, according to the Federal Bureau of Investigation.

The latest disclosures in the FBI-led investigation came as San Bernardino County employees began returning to work under tighter security, five days after Farook, an environmental health inspector for the county, and Malik opened fire with assault-style rifles on a holiday gathering of his colleagues.

The couple died in a shootout with police several hours after their attack on Wednesday morning in a conference room at the Inland Regional Center social services agency in San Bernardino, about 60 miles (100 km) east of Los Angeles.

The FBI said last week that authorities are investigating the mass shooting as an "act of terrorism," noting that Malik, a Pakistani native who lived most of her life in Saudi Arabia, was believed to have pledged allegiance on Facebook to the leader of the militant group Islamic State.

Posters of the 14 people killed are displayed on stage during a vigil for San Bernardino County employees after last week's shooting in San Bernardino, California December 7, 2015. Photo: Reuters

If the mass shooting - the deadliest burst of US gun violence in three years - proves to have been the work of people inspired by Islamic militants, it would mark the most lethal such attack in the United Sates since September 11, 2001.

In addition to five firearms recovered by investigators, authorities also have seized thousands of rounds of ammunition amassed by the couple, along with explosives and other materials for making as many as 19 pipe bombs, the FBI said.

Mounting signs that extremist ideology played some role in Wednesday's attack continued to reverberate in the campaign for the November 2016 US presidential election.

A day after Democratic President Barack Obama urged Americans in a televised White House address to avoid scape-goating of Islam as a religion, Republican presidential front-runner Donald Trump called for a ban on Muslims entering the United States.

SUSPECTS BELIEVED RADICALIZED 'FOR SOME TIME'

Questions have been raised about the extent to which Farook, who was born in Illinois to Pakistani immigrant parents and grew up in Southern California, might have been introduced to extremism by Malik, whom he married in Saudi Arabia in the summer of 2014 before returning together to the United States.

"The answer is we still do not know," said David Bowdich, assistant director of the Federal Bureau of Investigation's Los Angeles office. But, he added, "We have learned and believe that both subjects were radicalized and had been for quite some time."

Malik's transformation began before she came to the United States, according to the FBI. But Bowdich said it remained to be seen whether the husband and wife were indoctrinated by other individuals or whether they turned to extremist ideology on their own.

Two US government sources familiar with the case said on Monday that investigators had uncovered electronic communications indicating the couple had at least tried to contact militants abroad, but those communications were believed to have been part of a self-radicalisation process.

One source told Reuters the probe was focusing closely on contacts the shooters may have had with radical Islamists in the United States, rather than oversees.

Addressing that aspect of the probe in a news conference on Monday, Bowdich said. "I want to be crystal clear here. We do not see any evidence so far of ... an outside-the-continental-US plot. We may find it some day, we may not. We don't know."

While the couple may have been inspired by Islamic State, US government sources last week said there was no evidence their attack was directed by the militant group or that the organization even knew who they were.

FBI Director James Comey said on Friday that no information had been uncovered suggesting the killers were part of an extremist cell or network.

Asked about media reports that Farook may have come to the attention of law enforcement before Wednesday's killing, Bowdich said: "What I can tell you is we did not have an open investigation into  Farook at the time of this incident." He also said the FBI was working with foreign counterparts to expand the scope of its investigation.

To date, he said authorities have conducted well over 400 interviews in Southern California and collected more than 320 pieces of evidence.

SUSPECT'S MOTHER STILL BEING QUESTIONED

The FBI, he said, was continuing to seek a motive for the attack. Agents believe the couple had been planning more violence because of their cache of ammunition and explosives found in a bomb-making workshop in the suspects' home.

Farook's mother was still being questioned. She shared the couple's rented home in the town of Redlands and was caring for the suspects' 6-month-old daughter on the morning of the shooting. Officials have said the infant has since been placed in protective custody.

John D'Angelo, a special agent for the US Bureau of Alcohol, Tobacco, Firearms and Explosives, confirmed that the two rifles used in the attack were purchased by an individual named Enrique Marquez, described by several of his neighbors as having been a good friend of Farook.

Federal agents raided the Marquez home in Riverside, California, on Saturday, but it was not clear whether Marquez himself had been questioned. Federal law enforcement sources told Reuters that Marquez, considered a key witness in the probe, had checked himself into a Los Angeles-area psychiatric facility in the immediate aftermath of the shooting.

The two handguns and a .22-caliber rifle recovered by investigators were bought by Farook. D'Angelo said all five guns were legally purchased originally from licensed gun dealers in California between 2007 and 2012.

The mass shooting and its possible connections to Islamic militants quickly found its way into presidential politics, with several candidates for the 2016 Republican nomination accusing Obama of hesitancy in linking Wednesday's bloodshed in California to international terrorism.

In a televised address from the Oval Office on Sunday night, Obama condemned the attack as "an act of terrorism designed to kill innocent people" while cautioning against fear-mongering against the Muslim community and overreaction to the militant threat at home.

On Monday, Trump called for a blanket halt to immigration of Muslim individuals to the United States. "Until we are able to determine and understand this problem and the dangerous threat it poses, our country cannot be the victims of horrendous attacks by people that believe only in Jihad," he said.