Federal authorities have doubled the reward to $2.5 million for information about the person who mailed four anthrax-tainted letters, believed to have caused the deaths of four people in the fall.

Officials said they will mail fliers to more than 500,000 people asking for their help.

The reward money includes $2 million from the FBI and the U.S. Postal Service and $500,000 from direct mailer Advo Inc.

"This individual responsible for this may well be a neighbor, may well be a work associate," said Kevin Burke of the United States Postal Inspection Service.

Burke said the fliers will be mailed to residents in Bucks County, Pa., and in central New Jersey, where the letters were processed at the Hamilton postal facility. Fliers, which include pictures of the tainted envelopes, will also be distributed at transportation hubs.

The FBI is hoping someone will recognize the handwriting or the type of embossed 34-cent envelop used to send the bacteria. Officials believe the culprit has a scientific background, knows anthrax and is familiar with the Trenton area.

Five people in the U.S. died from inhalation anthrax last year — four of the deaths were believed to have been connected to the four tainted letters — and more than a dozen were infected with the bacteria. Most victims were postal workers or people working at media offices in New York and Florida, where detectives have confirmed or suspect that anthrax letters were sent.

"We're reaching out to you to ensure there are no more victims," said FBI Special Agent Kevin Donovan, who heads the Newark office.

Letters postmarked Trenton from the Hamilton facility were sent to NBC anchor Tom Brokaw, the New York Post and U.S. senators Tom Daschle and Patrick Leahy, forcing the closure of the nine-story Senate Hart Office Building for three months. It reopened Tuesday, after it was fumigated with a chlorine compound.

The Hamilton post office has been closed since last year.

Burke also stressed that postal officials have devoted many resources to finding the sender of the letters, including many tips that have turned out to be false.

Since the first cases of anthrax were identified last year, Burke said more than 15,000 anthrax hoaxes and threats have been reported to the postal service. More than 540 offices have been closed as a result and 71 people have been arrested, he said.

"If we find you, we'll arrest you and we'll prosecute you," Burke said.

The Associated Press contributed to this report.