NEW YORK (CNNfn) - A letter sent to juror No. 4 in the trial of the former CEO of Tyco International was written by a man in Boston who says he thought a mistrial had already been declared, and it had more of a "complaint" tone than a "threat" tone, New York Police Department sources told CNN.

Detectives and prosecutors in the office of Manhattan District Attorney Robert Morgenthau are investigating that letter and a phone call to Ruth Jordan, juror No. 4 in Dennis Kozlowski's trial. The letter was cited by Judge Michael Obus as exerting "outside pressure" on Jordan and was the ultimate reason the judge declared a mistrial.

A spokeswoman for the district attorney declined to comment.

The police department sources said the Boston man told NYPD detectives that he thought a mistrial had already been declared when he wrote the letter.

Those detectives have now referred the matter back to the district attorney, who will determine if any charges are warranted.

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An NYPD spokesperson had no knowledge as to the nature of the phone call to Jordan that is also being investigated. The district attorney's office would not comment on what is known about the phone call and the caller.

Jordan was the central figure in a dramatic and bizarre series of events that began with infighting among the jurors, included a widely reported gesture she made in court that appeared to be an "OK" sign toward the defendants. It ended with the judge granting a mistrial after Jordan's name had been reported by several media outlets and she had received a letter exerting "outside pressure."

The Manhattan district attorney's office has said that it will seek to retry Kozlowski and his co-defendant, former Tyco CFO Mark Swartz.

The office scoffed at the notion of accepting a plea bargain but would not comment on exactly which of the 32 counts would be recharged. Kozlowski and Swartz had been charged with conspiring to loot Tyco and its shareholders of $600 million worth of bonuses, special loans and stock grants.