Binghamton, N.Y., police released these undated photos of Jiverly Wong, the gunman who killed 13 and himself on April 3.
April 3: Authorities stand watch at the scene of a shooting in Binghamton, N.Y. that left at 14 people, including the gunman, dead.
April 3, 2009: SWAT teams surround an immigration center in Binghamton, N.Y., where a gunman opened fire and took hostages.
A Google map showing where a rampage and hostage situation at an immigration center in Binghamton, N.Y., unfolded
A map showing the location of the April 3, 2009, rampage at an upstate New York immigration center that left 12 dead plus the gunman
The police chief in Binghamton, N.Y., said Tuesday there is no reason to doubt the authenticity of a chilling letter purportedly sent by the troubled gunman who shot 13 people dead at an immigration center before killing himself.
Police Chief Joseph Zikuski said the rambling letter had been sent to behavioral experts for analysis, but detectives are certain it was written by Jiverly Wong, 41, the Vietnamese immigrant who committed Friday's horrific massacre.
"We have no reason to believe it's not his," Zikuski told reporters at a Tuesday news conference. "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 disjointed letter mailed the day of the massacre, Wong writes that he believed police had harassed him for years, even spreading rumors about him and touching him in his sleep, and said he was intent on killing people before returning "to the dust of the earth."
He ended the letter, neatly written in capital letters, on an ominous 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. He declined to name the shooting range, but said it was one of two in the area.
The chief said it was clear that Wong had psychological problems.
"There are obviously some mental health issues there," he said. "We saw some religious overtones. It's a good piece of evidence that's going to help us determine what happened, we hope."
Zikuski, appearing with New York senators Charles Schumer and Kirsten Gillibrand, declined to comment on specifics of Wong's state of mind.
"I am not a mental health expert," the chief said, adding he didn't know whether Wong had ever been treated by a psychologist or psychiatrist.
He said he will leave that assessment to the FBI experts.
Schumer and Gillibrand said they needed more information to determine whether state gun laws should be tightened, but spoke about a bill they sponsored making psychological evaluations part of prospective gun owners' background checks.
"We voted on a bill just last year to make sure mental health records are part of background checks," Gillibrand said. "Obviously, people with great mental illness should not have access to weapons."
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.
FOXNews.com's Catherine Donaldson-Evans and The Associated Press contributed to this report.