For nearly a year, the FBI and other law enforcement agencies probed scores of threatening letters from the same source promising to kill elected officials and bomb targets across the country, including the Mall of America.

One wave of letters sent to a Minneapolis resident and to multiple former presidents last year vowed to copy the Oct. 1 mass killing of 58 people in Las Vegas: "MORE COMING YOUR WAY ….."

"Better watch out for these people," read another note sent to the White House a week later in one of the writer's frequent references to a network he called the Unstoppable Force. "They are planning to do it for the next concert, just like the Las Vegas shooting … big gun and pipe bomb."

But by then investigators had closed in on a 34-year-old man who recently moved from Minnesota to the Sacramento area, where he worked long hours at a distribution center for a cracker company. Before agents arrested him, Kao Xiong allegedly wrote more than 150 letters, some of which contained white powder later tested to be flour.

Xiong is now back in Minnesota, staying with relatives under house arrest ahead of his next court date in January in California, where he is expected to plead guilty to multiple charges of conveying false information concerning the use of an explosive.

Xiong spent six months in custody in California before a defense attorney successfully lobbied for his release earlier this year over prosecutors' objections, who pointed to mental illness and argued that Xiong never intended to act on any threats he made.

"Mental breakdown, that's all it was," Tim Zindel, an assistant federal defender representing Xiong, said in an interview. "Just a crazy person, not a dangerously crazy person. He's actually very sweet."

Xiong is a Hmong refugee born in a refugee camp in Thailand in 1984. He immigrated to Oroville, Calif., with his family. He moved with his parents to Minnesota in 1999 before returning to Oroville after an "emergency psychosis episode" following his marriage's breakdown, according to Zindel.

Prosecutors have argued that Xiong is still a public safety threat and flight risk. The federal magistrate judge who granted Xiong's release this year also ordered him to turn over his passport and to wear an ankle monitor while out on bail.

"I have FBI watching myself," Xiong wrote on Facebook, posting a photo of his ankle monitor in July.

According to an FBI agent's affidavit filed last year, Xiong allegedly composed "poison-pen style" letters that identified his estranged wife and another Minnesota man as responsible for the threats to try to draw law enforcement toward them.

"The letters have resulted in the expenditure of significant government resources and have victimized innocent civilians," FBI special agent Scott Wales wrote, adding that most of the letters required processing through a lab and careful analysis from someone trained in weapons of mass destruction.

Wales wrote that Xiong started sending the letters in January 2017, sending notes threatening President Donald Trump and former Presidents George W. Bush and Barack Obama and a bomb threat letter to the Dallas/Fort Worth International Airport.

U.S. Secret Service agents and the FBI each separately interviewed Xiong's estranged wife last year. She said Xiong previously accused her of having a relationship with the other person he named in the letters and that he had started using meth around the time they split up in 2015, according to court papers. She also said she had to get a restraining order.

Xiong denied knowing about the letters when agents spoke with him in February 2017, after which he allegedly continued mailing off a torrent of threats: He promised to pipe bomb the Minneapolis Social Security Office in May 2017 and sent identical letters threatening to "BLOW THIS BUILDING" to the Minneapolis-St. Paul International Airport, the Mall of America and the Hennepin County Human Services and Public Health Department a month later.

After Xiong allegedly sent bomb threats to the offices of Yahoo, the New York Times and Myspace, FBI agents interviewed him at his job. Xiong again denied sending the threats, and one of the agents left behind a business card.

That agent's name and work address began appearing in threat letters sent in November and in the weeks before Xiong's December arrest. One letter described making "pipe bomb for New Years," named that agent and said, "His family is NEXT."

Agents later followed Xiong's car to a law library, where they found copies of Microsoft Word documents named "UNSTOPPABLE FORCE" and "FBI ADDRESS" on a computer. Video surveillance also captured Xiong purchasing postage stamps, stationery and disposable vinyl gloves. Agents followed Xiong to two mailboxes in Oroville and pulled 78 of his letters from the boxes. They tracked him from work, to the gym and to weekly visits to Buffalo Wild Wings, which he often documented on Facebook.

"No more hot wings," Xiong wrote earlier this year after a friend asked what he was up to.

Before his arrest, Xiong sent another round of letters to FBI field offices across the country, claiming in one of the notes that his parents were responsible for his actions and that he was the victim of a framing attempt.

Zindel has said that a psychiatrist has since diagnosed Xiong with "an unspecified psychosis" and that he now appears to be in remission.

The FBI declined to comment for this story. A spokeswoman for the U.S. Attorney's Office in Sacramento said Xiong's charges carry a maximum sentence of 10 years in prison for each count. But Zindel said that Xiong's plea agreement would likely result in no further incarceration.