Federal agents are probing the background of a former US Marine for allegedly plotting a Christmas attack in San Francisco inspired by the Islamic State jihadist group, according to court documents.

Tow-truck driver Everitt Aaron Jameson, 26, was planning to target the city's busy Pier 39 tourist spot, according to an affidavit submitted Friday by FBI Special Agent Christopher McKinney.

The suspect is said to have outlined to undercover agents a plan to use explosives to target crowds at the pier -- which is lined with restaurants, shops and bars -- between December 18 and 25 because "Christmas was the perfect day to commit the attack."

Jameson professed not to need an escape plan as he was "ready to die," the document said. As a former soldier, he said, "I have been trained in combat and things of war."

The FBI said the public was "never in imminent danger."

Yet McKinney acknowledged an FBI misstep that could have endangered the investigation.

He said that on Monday, an FBI employee using a telephone number with a Washington DC area code had mistakenly called Jameson's cell phone. When Jameson answered, apparently in Arabic, the employee immediately hung up.

When Jameson called back, he got a voicemail message giving the employee's name but not identifying the agency.

That evening, Jameson told his undercover contact, "I don't think I can do this after all. I've reconsidered."

FBI agents raided the suspect's home in Modesto, California two days later, allegedly finding his last will and testament along with a .22-caliber Winchester rifle, handguns, fireworks and ammunition.

Jameson attended basic training with the Marine Corps in 2009, obtaining a "sharpshooter" rifle qualification, the FBI said, but was discharged after failing to disclose a history of asthma.

According to McKinney, Jameson selected Pier 39 because he "knew it was a heavily crowded area."

- 'Radical jihadi beliefs' -

"Jameson explained that he also desired to use explosives" in an attack that could involve his tow truck as an additional weapon, McKinney stated.

The suspect revealed his plans to an undercover FBI agent he believed to be a senior leader of the Islamic State group (IS), according to the court document.

Jameson said the US needed "another attack like New York or San Bernardino," involving vehicles and firearms.

He had voiced support for the October 31 attack in New York in which a jihadist drove a pickup truck into a crowded bike path, killing eight. He allegedly had messaged "I'm glad to know we Muslims are finally hitting back," the FBI said.

In the 2015 attack in San Bernardino, California, a married couple used firearms to kill 14 people at a government building.

According to the 11-page criminal complaint, Jameson "has espoused radical jihadi beliefs." He had "liked" pro-IS posts on Facebook, including an image of Santa Claus standing in New York with explosives.

In a letter seized at his home, presumably intended to be found after his death, Jameson wrote that "You've allowed Donald J Trump to give away Al Quds (Jerusalem) to the Jews," a reference to the president's controversial decision to recognize Jerusalem as the capital of Israel.

- 'Gentle, kind' -

He was charged in the Eastern District Court of California with attempting to provide material support to a designated foreign terrorist organization and faces up to 20 years in prison if convicted. A preliminary hearing is slated for January 5.

Local newspaper the Merced Sun Star published a video interview with the suspect's father, Gordon Jameson, who described his only child as "the gentle, kind type of Muslim person."

"The FBI isn't saying much to me, but I know my son wouldn't harm nobody. He wouldn't do that to innocent people," he said, adding that he had been told his son was on suicide watch.

The Sacramento Bee quoted the grandfather of Jameson's ex-wife Ashley Monett Jameson as saying the couple have two young children.

The paper said Ashley Jameson is in prison -- the reason was unclear -- and the children are in foster care.

Gordon Jameson said his son was "devastated" after being told he would never regain custody of his children.

The FBI affidavit describes Everitt Jameson as messaging an undercover agent just 10 days ago to assure him and his ostensible jihadi superiors that he was ready to act.

"Tell them anything... I am ready. Give the word and it shall be done."