Mercer County Sheriff Kevin C. Larkin said the letter was addressed to the mayor, but not by name.
"There was a letter addressed to the mayor of Princeton and a couple of people who touched it, including her secretary who opened it, displayed blisters and rash-like symptoms on their face and some people complained of nasal passages being blocked," Larkin said.
There were about 30 employees in the building at the time of the incident, but no one had to be decontaminated, officials said. The building was evacuated and workers sent home for the day.
Federico said some people began experiencing minor lip burning after the first reaction took place, but that officials haven't been able to determine what caused the effects.
"We don't believe it's serious," he said. "If anything, it was an irritant."
The letter, in an envelope mailed inside a larger envelope, was tested onsite and nothing radiological or chemical was found, Larkin said. The letter was going to the state health lab for further tests.
Larkin said investigators have spoken to the author of the letter, a resident of a Louisiana nursing home.
"He's a 70-year-old guy confined to a wheelchair who spends a lot of time on the Internet," Larkin said. Authorities would not release the man's name or say where in Louisiana the nursing home is.
Larkin said the man had suffered some kind of brain trauma and believed he could get help from a research center in the area. "He was trying to contact the center asking for vitamins and other information," he said.
Larkin said he was unsure if there is such a center in Princeton Borough, which is home to Princeton University, across the street from the municipal complex.
Larkin said the investigation was continuing and he did not know if any charges would be filed.
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