WASHINGTON (Reuters) - They were a married couple with a young child and a steady income. Those who knew them said they didn't talk about the Islamic State in Iraq and Syria (ISIS), even as they were amassing an arsenal of pipe bombs and assault weapons. They kept a low profile on social media.

The husband and wife accused of killing 14 people in California bore little resemblance, apart from their Muslim faith, to the aimless young men who have been arrested in the United States for plotting violent attacks in the name of ISIS.

At the time that Syed Rizwan Farook, 28, and Tashfeen Malik, 27, were shooting people at an office holiday party in San Bernardino, California, the Federal Bureau of Investigation (FBI) was investigating more than 900 United States residents for suspected ties to Islamic extremist groups.

Malik and Farook do not appear to have been on the list, raising major questions about the effectiveness of a massive law-enforcement effort meant to head off such attacks, including whether there need to be changes in the current strategy.

The FBI is investigating the incident on Wednesday as an act of terrorism. If the two are found to be Islamic militants, it would be the most serious such attack since the Sept 11, 2001 destruction of the World Trade Center and attack on the Pentagon.

It comes less than a week after President Barack Obama and other administration officials said they were unaware of any significant ISIS-inspired threats to the US.

FBI officials said it was too soon to reassess their counter-terrorism strategy, with investigators still trying to piece together what motivated Farook and Malik, and whether authorities could have done anything to stop them.

"We're too busy to do that right now, but we will eventually look back to see is there learning here for us," FBI Director James Comey told reporters on Friday (Dec 4).

The labeling of the San Bernardino attack as a terrorist act was expected to fuel debate in Washington about whether the FBI, National Security Agency (NSA) and other intelligence agencies have adequate powers and resources to track and hunt down violent extremists who don't have clearcut profiles.

Complaints about the intrusiveness of the NSA led just a week ago to the shutting down of its daily vacuuming of millions of Americans' phone records. This step, a victory for privacy advocates, was opposed by some in Congress who called the surveillance a key element of national security.

There may also be questions about whether US vetting of those applying for visas is tight enough to prevent someone like Malik from getting into the country. She entered the United States on a fiancée visa in 2014. Lawmakers and Mr Obama have both said they will address the visa issue.

But US officials say there is simply too much material and too many suspects to monitor to be able to foil every single attack.

Given their number, it is difficult even to keep close track of the suspects clearly connected to ISIS, let alone those who have not been vocal about their sympathies, either online or in the real world.

The best bet, Mr Comey said, is for citizens to report suspicious activity. "What we hope you would do is not let fear become disabling, but instead try to channel it into an awareness of your surroundings," he said. "If you see something that doesn't make sense, you say something to somebody."

FBI officials say ISIS poses a different sort of threat than Al-Qaeda and other extremist groups because it inspires sympathizers to act on their own, leaving little time between the "flash" of radicalisation and the "bang" of an attack.

Over the past two years, the FBI has arrested at least 71 people for suspected ties to the extremist group.

According to a Reuters review of court documents, those who have been charged have tended to be young men. They have often expressed their support for ISIS on Facebook, Twitter and other types of social media, allowing FBI agents to identify and track them.

Typically, the agency assigns undercover agents or informants to draw out the suspects, gathering further evidence on their plans and sometimes supplying them with bomb-making material and other equipment for an attack that is then stopped before there is a risk to the public. This process can take months.

Mr Comey said ISIS frequently strikes a chord with young people with personal problems. "We've seen cases over the last nine months, people with drug problems, people with family problems, people with mental health problems. People who are struggling and trying to find a centre in their life," he said.

Mr Comey said the number of open cases has declined over the last six months.

Some 46 of the 71 arrested so far aspired to wage jihad overseas, not in the United States. More than half of those arrested were trying to travel to Syria to join the ISIS or had already made the trip, according to the Reuters review.

Farook and Malik at this point don't appear to have been on any sort of terrorism watch list, government officials said.

With a six-month-old child, a steady job as a health inspector and a large extended family, Farook showed little outward signs of the social isolation and violent talk that the National Counterterrorism Centre said can make individuals susceptible to extremist ideology.

An attorney for Farook's family said he was an isolated individual who had few friends, but the family had no idea that he was planning an attack.

Unlike many members of the Millennial generation, Farook and Malik left few digital footprints on the social-media outlets that have prompted many other FBI investigations.

FBI officials said Malik pledged allegiance to ISIS leader Abu Bakr al-Baghdadi in an online post online shortly before she and Farook stormed the party at a social services centre.

Facebook said it had taken down a page set up by Malik under an alias and is cooperating with investigators.

Mr Lorenzo Vidino, an expert with George Washington University's Project on Extremism, said his group found no trace of the couple in its archive of Islamic militant activity online.

While members of Al-Qaeda tend to be driven by political ideology, those attracted to ISIS have tended to be motivated by personal setbacks such as losing a job or getting in a fight, said Ms Karen Greenberg, director of the Fordham University's Center on National Security.

"If you read what they're saying, a lot of it is about themselves, not about a political agenda," she said.