WASHINGTON: California shooter Syed Farook plotted an attack with an accomplice three years ago but got "cold feet" after a wave of Unites States (US) counter-terrorism arrests, a senator said Wednesday.

A longtime friend and neighbour of Farook's, Enrique Marquez, told investigators of their aborted plot, said Senator James Risch, who sits on the Senate Select Committee on Intelligence.

"He has admitted that in 2012 they had something in mind and they didn't do it because there had been some immediate arrests by the counter-terrorism people," Risch told CNN. "And as a result of that they got cold feet."

The precise nature or target of the plot was unclear, Risch said.

Marquez, whose home was raided at the weekend, is suspected of purchasing two assault rifles used by Farook and his wife Tashfeen Malik during the terror rampage that killed 14 people at a year-end office party on December 2 in San Bernardino. The couple were later killed in a police shootout.

The 24-year-old Marquez bought the guns legally in 2011 or 2012 and then gave or sold them to Farook, federal officials told The New York Times.

Investigators are seeking to establish how much he knew of — or shared — the deadly intentions of the couple.

A supermarket employee and convert to Islam, Marquez is currently being treated in a psychiatric ward. He has yet to be charged in the probe and could potentially be offered a plea bargain if he cooperates.

Marquez and Farook were neighbours in the Californian town of Riverside for around a decade, according to The New York Times.

The two men were also loosely related by marriage, according to the Times, which said Marquez last year married a woman named Mariya Chernykh, who is a sister of Farook's sister-in-law.

The Federal Bureau of Investigation said that it believes Farook and Malik became radicalised before they met — first online, and then in person in Saudi Arabia.

Also read: Suspects Syed Farook, Tashfeen Malik kill 14 in California shooting: authorities