The FBI believes Tashfeen Malik posted a statement on Facebook using an alias before launching the sickening assault with husband Syed Rizwan Farook.
Sources close to the investigation confirmed that they had found a message of support to IS chief Abu Bakr al-Baghdadi – but did not explain how they knew Malik, 27, had made the post.
Others added that Malik expressed “admiration” for al-Baghdadi but there was no sign anyone affiliated with IS replied or that any operational instructions were passed to her.
One line of inquiry is whether she radicalised Farook, 28, who was a devout Muslim. But some sources said he had been in contact with at least two overseas terror groups, including the Nusra Front in Syria and al-Shabab in Somalia.
The FBI confirmed it is probing the massacre as an act of terrorism and the couple’s family have been contacted in Pakistan as they build up information on their movements.
Agents are looking into whether they received any weapons or terror training in the country.
The couple were shot dead in a fierce battle with police hours after they stormed the party at Inland Regional Centre in San Bernardino, where Farook had worked for five years, on Wednesday. Their attack also left 21 injured.
The news of the terror link came as images emerged showing the home-made pipe bombs the couple planned to use.
Crude devices were left behind at the scene of the massacre and defused by bomb squads, and a dozen more were recovered at the killers’ rented home. One image taken by a bomb squad robot shows a yellow toy car attached to pipe bombs and several wires. It was later blown up.
The couple had stockpiled an arsenal including two high-powered assault rifles, handguns and more than 6,000 rounds of ammo. Also inside the property were items for their baby, which they dropped off with its gran before the massacre. The fridge was fully stocked and the sink was full of pots and pans.
Photo IDs and a range of documents and photos were found alongside their child’s toys. The FBI said they found several shredded documents and two mobiles that had been destroyed.
Meanwhile, Farook’s friends said he stopped going to prayer three weeks ago. He had also recently had an argument over Islam with one of the co-workers he shot dead – Messianic Jew Nicholas Thalasinos.
Inside shooting suspect Syed Farook's home, in pictures:
Mr Thalasinos, 52, was passionate about pro-Israel causes and known to rant about Islam on Facebook.
His friend Kuuleme Stephens said she heard him rowing with Farook two weeks ago.