BINGHAMTON, N.Y. (AP) âÄî Police on Tuesday said they have no reason to doubt that a letter mailed to a TV station forecasting last week’s mass shooting at an immigrant center was written by the gunman. The letter received by Syracuse TV station News 10 Now is still being analyzed, but police believe it was written by 41-year-old Jiverly Wong, police chief Joseph Zikuski said at a briefing. “We have no reason to believe it’s not (authentic),” 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. ” In the rambling, disjointed letter mailed the day he gunned down 13 people before killing himself, 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.” The package was dated March 18, more than two weeks before the shooting. It included photos of Wong smiling with two guns, a gun permit and his driver’s license. The envelope carried three stamps: two Purple Hearts and a Liberty Bell. Zikuski said police are trying to determine if photos included with the letter were taken at a local shooting range. He said police heard that Wong had asked others at a shooting range to take pictures of him. Zikuski declined to discuss what police thought the letter says about the mental stability of gunman Jiverly Wong. He said he will leave that assessment to FBI experts. “There are obviously some mental health issues there, we saw some religious overtones,” Zikuski said. “They’re (the FBI) the experts to tell us exactly what that letter means.” Zikuski said the letter hasn’t gotten them any closer to figuring out why he targeted a classroom full of immigrants learning English. “We may never know that,” he said. 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. Zikuski said police received information earlier Tuesday that the letter might not be from Wong, but said that information turned out to be incorrect. He didn’t comment on the source of the information. 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 the letter, Zikuski said. 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. “I am Jiverly Wong shooting the people,” the letter says. The letter ends with him saying he can’t “accept my poor life” and will “cut my poor life.” Police have speculated Wong, who was ethnically Chinese but was from Vietnam, was angry over losing a job and frustrated about his poor English skills.