For two years, someone has been sending letters threatening to blow up two synagogues in White Oak and kill a rabbi and the local police chief, and now federal authorities are asking for help to find the culprit.

Yesterday at a news conference at the borough building, the FBI and U.S. postal inspectors offered a $10,000 reward for information leading to a conviction.

"We want to put an end to this," said Mayor Ina Marton. "We want closure."

FBI Agent Kevin Deegan and Police Chief Joe Hoffman said 10 letters have been sent in all, beginning in August 2004 with a threat to blow up Temple B'nai Israel.

The temple received another threat in December of that year.

Another synagogue, Gemilas Chesed, has received six letters, the most recent one arriving in June. Several of those contained specific death threats against Rabbi Irvin Chinn and his family. Chief Hoffman has also received two death threat letters.

Federal authorities would not describe in detail the specific threats, but said they are explicit and express hatred for Jews. Several have contained a white powdery substance that turned out to be flour or a similar product.

It's not clear why the police chief was targeted, since he's not Jewish, but he said it was probably because he held a news conference shortly after the first letter arrived to alert the Jewish community about the threat.

"White Oak police are part of the investigation, and I'm visible," he said. "I think that had something to do with it."

Still, the timing was strange. Chief Hoffman held his news conference in September 2004 but didn't get his first threat letter, complete with powder, until May 2005.

Investigators don't know why the sender waited eight months to threaten the chief.

"That's a question for the writer," said Postal Inspector David Bosch.

Rabbi Chinn did not return a message seeking comment.

Authorities would not discuss any potential suspects or leads, but they do have some clues.

Inspector Bosch said the sender used a specific series of stamps with pictures of antique toys on them that the post office produced in 2003.

"What that tells you is that this is probably a person who doesn't mail frequently," he said, because the stamps were bought in 2003 but not used until 2004 and 2005. He said the buyer might have specifically requested those stamps and bought them in bulk.

Inspector Bosch also said investigators believe the suspect is likely trying to disguise his or her handwriting, based on FBI analysis of the letters.

But finding out the location from where they were mailed is more difficult than it might seem.

Mail sent from White Oak to another address in White Oak doesn't stay in the borough. Instead, like mail throughout the region, it travels to the main processing facility in Pittsburgh and is stamped with a cancellation mark that says "Pittsburgh" -- just like tons of other mail.

Inspector Bosch said the best chance of solving the case is a tip from the public.

"People talk," he said. "Somebody out there knows something."

The first letter, postmarked Aug. 30, 2004, caused a stir in the community. Temple B'nai called police, who called the Joint Terrorism Task Force. A hazardous materials unit arrived to take away the letter, which contained a threat to blow up the synagogue.

Since then, these are the dates of the other postmarks and their targets:

•Dec. 6, 2004: Gemilas Chesed

•Dec. 8, 2004: Temple B'nai

•May 27, 2005: Chief Hoffman

•July 11, 2005: Gemilas Chesed

•Oct. 18, 2005: Gemilas Chesed

•Nov. 1, 2005: Gemilas Chesed

•December, 2005: Gemilas Chesed (date smudged)

•Dec. 20, 2005: Chief Hoffman

•June 9, 2006: Gemilas Chesed

Anyone with information about the letters is asked to call the FBI at 412-432-4000. Those who wish to mail information are asked to send it to P.O. Box 101461, Pittsburgh 15237.