BINGHAMTON, N.Y. (AP) — A rambling letter from the gunman who killed 13 people at an immigrant community center offers a glimpse inside the mind of an angry, paranoid man who saw himself as a victim and was intent on destroying other lives before he took his own, analysts say.

It’s hard to say, though, whether friends or relatives would have noticed the demons that tormented 41-year-old Jiverly Wong leading up to Friday’s massacre at the American Civic Association.

Police have speculated that Wong, who was ethnically Chinese but was from Vietnam, was angry over losing a job and frustrated about his poor English-language skills.

Authorities said Tuesday that they have no reason to doubt the letter was written by Wong. It was mailed to Syracuse TV station News 10 Now, complained of supposed harassment by police and grimly foretold of the mass murder.

“We have no reason to believe it’s not (authentic),” Police Chief Joseph Zikuski said. “It’s another piece of evidence in a very complex puzzle that’s going to take us weeks and months to determine what’s going on. It’s going to help us.”

A former FBI profiler who reviewed the letter called it a classic rant from a mass murderer.

“Everybody is out to get him, and the world is a threatening place,” said Gregg McCrary, who is now an author and consultant based in Fredericksburg, Va.

In the disjointed letter mailed the day he opened fire inside the center, Wong blamed his troubles on the police and vowed to take at least two people “to return to the dust of earth.” He ended the letter, neatly written in capital letters, on a chilling note: “And you have a nice day.”

“Every mass murder we’ve ever studied has had a combination of paranoia … depression and suicidality,” said Park Dietz, who is president of Park Dietz & Associates Group Inc., a forensic litigation firm, and Threat Assessment Group Inc., a Newport Beach, Calif., violence prevention firm.

“This (letter) makes it clear how severe that paranoia was in this case,” he said.

The letter, dated March 18, more than two weeks before the shooting, was accompanied by photos of Wong smiling with two guns, a gun permit and his driver’s license.

Zikuski declined to discuss what police thought the letter said about Wong’s mental state. He said he will leave that assessment to FBI experts.

“There are obviously some mental health issues there,” Zikuski said.

After the letter surfaced Monday night, some community members wondered whether it should be harder to get a pistol permit.

“Quite frankly, I think they should do away with pistol permits on such people,” said Arlene Ryant, whose granddaughter had one of the victims, Roberta King, as a substitute teacher. “They should do a more thorough background (check) to make sure they don’t have any mental illnesses.”

Federal law prohibits anyone with a court record of mental illness or outpatient mental health treatment from owning a gun, and Wong had no such records.

Zikuski said the letter hasn’t gotten them any closer to figuring out why Wong, who had used the American Civic Association as a resource, targeted a classroom full of immigrants learning English there.

“We may never know that,” he said.

The letter indicated he was angry with police for what he said were years of persecution. But records show few scrapes with the law, save for a bad check charge in California in 1992 and a speeding ticket there in 2007. An informant told state police that Wong was planning a bank robbery in 1999 to support his cocaine habit, but Zikuski said the information was never verified.

The chief was not aware that Wong had ever been put under surveillance by authorities in New York or California, as he claimed in the letter.

Wong’s sister said on NBC’s “Today” show she doesn’t think the letter was written by him. The woman, who asked not to be identified, said her younger brother’s handwriting was more like “chicken scratch” and his vocabulary too limited to have written such a letter.

DNA testing on the letter has not yet begun because police have to collect samples from the five or so News 10 Now employees who handled it, Zikuski said. But he noted that information earlier Tuesday that the letter might not be from Wong turned out to be incorrect.

Family members might not pick up on warning signs when someone becomes paranoid, so it’s wrong to suggest they could have prevented the carnage, McCrary said.

“The paranoia makes them very distrustful so they tend to become reclusive,” he said. “In hindsight, it’s clear. You can go back and pick up the warning signs, probably. But family members and those around him are just going to think he’s just a little weird, or troubled.”

McCrary said the rampage could be viewed as “one big suicidal event.”

“He’s going out, but he’s taking people out with him, and he’s going out with a huge detonation,” he said. “He’s going to get all of our attention.”

At the Court Jester athletic club, where Wong played racquetball and used a treadmill in the weeks before the shootings, Norm Donahue, 62, said he chatted with him two or three times.

“I would just say hello to him; I never really got any bad vibes,” said Donahue, a retired school administrator who organizes the club’s racquetball league.

Patrick Ranger, 28, lives across the street from the house where Wong lived with his parents and sister and met him occasionally, as neighbors do, “taking the garbage out, shoveling snow.” He said Wong’s behavior seemed to change in recent weeks.

Wong had “the same demeanor as if you were going through a divorce,” said Ranger, a machinist with a wife and 16-month-old daughter. “Like, he would walk with his head down sometimes and not upbeat and everything, but not where you would be like, ‘The guy’s losing his marbles.’ More like, the same as everyone who’s having a bad day.”

The letter was mailed from Binghamton and postmarked Friday, the day Wong went into the American Civic Association community center and started shooting. Two employees and 11 immigrants taking an English class died in the assault.

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Associated Press writers Ben Dobbin and John Kekis in Binghamton contributed to this report.