FBI agents have searched the home of the man believed to have bought two assault rifles used in the killing of 14 people at a social services centre in San Bernardino.
The address in Riverside, California raided on Saturday was next door to where the couple who carried out the attack - Syed Farook and Tashfeen Malik - used to live.
Police could not talk to Enrique Marquez because he checked himself into a mental hospital after the attack, according to Associated Press.
Islamic State has praised the attacks and the 28-year-old US-born Farook is believed to have had contact with militant suspects. His Pakistani wife, 29, reportedly put a statement on social media about IS shortly before the killings.
Mr Marquez is believed is understood to have bought the assault rifles about three years ago but is reportedly not considered a suspect.
It is not clear whether he purchased the guns for Farook or whether Farook obtained them from him later.
During the raid, agents cut open the garage door with blow torches, used bomb-sniffing dogs, and temporarily detained his father and younger brother, NBC reported.
Neighbour Freddy Escamilia told AFP news agency: "I heard the windows crashing, the garage door being torn, doors being broken down, you know, they were forcing their way in."
Despite IS describing the couple as "soldiers" of its self-proclaimed caliphate and some security officials in the US indicating the couple may have been radicalised, the FBI says there are no signs they were members of the extremist group.
Relatives of the couple also say that, while they were devout Muslims, there was no sign they had become radicalised and were simply a quiet couple who kept to themselves.
But David Bowdich, assistant FBI director in charge of the Los Angeles office, said officials are examining a Facebook post in which Malik is believed to have pledged allegiance to IS leader Abu Bakr al-Baghdadi around the time of the attack.
Farook and his wife, who were killed in a police shootout, had a six-month-old daughter who they left with Farook's mother before carrying out their attack.
Meanwhile, US President Barack Obama will make a rare primetime TV address telling Americans how he will keep them safe following what will have been the deadliest terror attack since September 11, 2001, if terror links are confirmed.
The last time the president made such an address was in August 2010, to mark the end of US combat operations in Iraq.
"(The President) will reiterate his firm conviction that ISIL will be destroyed and that the United States must draw upon our values - our unwavering commitment to justice, equality and freedom - to prevail over terrorist groups that use violence to advance a destructive ideology," the White House said.
