Heath, whose career has included several controversies, contends that she's facing retribution for filing a complaint against an Internal Affairs higher-up.
The sources said some letters were mailed to top cops in the department and others were sent to Internal Affairs.
One letter contained a forged signature of a retired officer, AnnaMae Law, the sources said.
"We did have some assistance from the FBI," Police Commissioner Charles Ramsey told the Daily News recently.
"There was enough there for Internal Affairs to sustain the case."
Ramsey said a chief inspector and two deputy commissioners will hear the case and will recommend a course of action.
"She'll get a fair hearing," he said.
Heath, who joined the force in 1981, denied writing the letters.
"The allegations are not true," she said last night. "Just because someone in the Police Department sustained an allegation doesn't mean that it's true. I would have no reason to write those letters."
She said Thomas Thompson, a retired captain, penned the letters because he was upset about how discipline is handled in the department.
Thompson could not be reached for comment.
"We've been friends for well over 20 years," Heath said, "but I was not aware that he was doing that."
Heath said Thompson informed John McNesby, president of the Fraternal Order of Police Lodge 5, of his involvement.
McNesby said last night that he fielded a phone call from Thompson about the incident several months ago, and forwarded the information to Internal Affairs.
It's unclear whether Thompson went on the record with Internal Affairs investigators, or how much it would matter if the FBI has determined that the handwriting is Heath's.
"If I was so inclined to write the letters - which I'm not - it's ridiculous to think that I would do it in my own handwriting," Heath said, noting that she used to oversee the Forensic Science Bureau.
"I just know that Internal Affairs made a determination, and the case has been sustained," Ramsey said.
Heath said she filed an Equal Employment Opportunity complaint in October against Internal Affairs Chief Inspector Chris Flacco over the way that he handled Internal Affairs complaints lodged against her.
Soon after, she said, she received notice that she would be disciplined over the investigation into the letters.
In 1998, she received a $150,000 settlement from the city after she alleged in a lawsuit that she had been sexually harassed by a supervisor in the early '90s.
Heath went on to become the first female chief inspector in the Police Department.
In 2006, then-Commissioner Sylvester Johnson fired Heath after Internal Affairs investigated allegations that she had stalked and harassed an ex-boyfriend and his wife, both cops in the K-9 Unit.
The couple filed a lawsuit against the city a year later.
Heath won her job back through arbitration.
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