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.
Investigators believe the married couple who massacred 14 people in California last week - a U.S.-born husband and his Pakistani wife - had been radicalized "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.
If the mass shooting - the deadliest burst of U.S. 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 Sept. 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 U.S. 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.