Jameson allegedly told the agent he had been a soldier, and "I have been trained in combat and things of war."
He appears to have graduated from U.S. Marine Corps recruit training in late 2009, earning a "sharpshooter" rifle qualification, according to the affidavit. He was ultimately discharged from the Marine Corps, however, for failing to disclose a history of asthma.
Another undercover agent arranged a meeting with Jameson last weekend and intimated that he was a high-level operative for the Islamic State.
"Jameson provided a positive response and appeared pleased" the person he was meeting was affiliated with the Islamic State group, according to the affidavit.
During the conversation, he allegedly offered to donate $400 per month and said he was willing to travel to Syria.
"On four separate occasions, the [undercover agent] encouraged Jameson to go home and think about what he was saying," the affidavit says. "But Jameson rebuffed the [agent's] requests, maintaining that he was ready and prepared to proceed."
He allegedly suggested another attack like the one in New York and in San Bernardino in 2015.
"Jameson specifically named Pier 39 in San Francisco as a target location because he had been there before and knew that it was a heavily crowded area," the affidavit says. "Jameson explained that he also desired to use explosives, and described a plan in which explosives could 'tunnel' or 'funnel' people into a location where Jameson could inflict casualties."
It continues: "Jameson also stated that Christmas was the perfect day to commit the attack."
He allegedly asked the agent to provide assault weapons and timers or remote detonators. But he offered to get additional items to craft homemade bombs.
The agent asked Jameson to make a video or written statement "for the brothers," according to the affidavit, and Jameson responded later that day, "It is written."
On Monday, Jameson allegedly sent the agent photographs and a map of the Pier 39 area.
The affidavit says an FBI employee mistakenly called Jameson's cellphone later that day from an identifiable, Washington, D.C., phone number.
"After Jameson answered in apparent Arabic language, the FBI employee immediately terminated the call," the affidavit says. "Shortly afterwards, Jameson called the 202 telephone number, which resolves to a voice mail identifying the name (but not employing agency) of [the] user of the 202 telephone."
Later on Monday, Jameson appeared to try to back out of the plot.
"Jameson told [the undercover agent], 'I also don't think I can do this after all. I've reconsidered,' " according to the affidavit. "'In Sha Allah one day I can. But I can't.' "
The FBI served a search warrant at Jameson's Modesto home on Wednesday, where agents recovered two rifles, a handgun and ammunition, as well as a will allegedly executed by Jameson in November and the letter an undercover agent asked him to write.
"You all have brought this upon yourselves," the letter allegedly states. "We have penetrated and infiltrated your disgusting country. These Acts will continue until the Lions of Islam overtake you."
The charge of attempting to provide material support to a terrorist organization carries a maximum 20-year prison sentence, according to the U.S. Attorney's Office for California's Eastern District, which is prosecuting the case.
Attempts to reach someone at Jameson's home were unsuccessful. It's unclear if he is represented by an attorney.
"Today, our incredible law enforcement officers have once again helped thwart an alleged plot to kill Americans," U.S. Attorney General Jeff Sessions said in a written statement.
Acting San Francisco Mayor London Breed said in a statement that there are no additional known threats to the city, and local police officers will increase their presence.
"Thanks to the diligent efforts of the FBI and local law enforcement agencies, a planned attack has been prevented from occurring in our city," Breed said. "We will not allow the thwarted attempts of one dangerous individual to disrupt our way of life. We will remain vigilant and continue to protect our city from any threat."
Read the criminal complaint and affidavit below.