EWING, N.J. - After four months without an arrest in the case, federal authorities yesterday doubled the reward to $2.5 million for information leading to the capture of whoever sent four letters tainted with anthrax.

Officials also said they will send fliers to more than 500,000 people in New Jersey and Pennsylvania asking for their help.

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

The fliers, which include pictures of the tainted envelopes, will be sent to residents in Bucks County, Pa., and in central New Jersey, where the letters were processed at the Hamilton postal installation.

The FBI hopes someone will recognize the handwriting or the type of embossed, 34-cent envelope used. Officials believe the culprit has a scientific background and is familiar with the Trenton area.

"We're reaching out to you to ensure there are no more victims," FBI agent Kevin Donovan said. "I don't believe this person is as elusive as the Unabomber."

He added: "It's not that we don't have leads. We're looking for that person out there who might have information."

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

Five people died of anthrax last year - including two postal workers - and more than a dozen people were infected.

The tainted letters - sent to NBC anchorman Tom Brokaw, the New York Post and two U.S. senators - were postmarked in Trenton in September and October and went through the Hamilton facility. Officials believe other people were infected through cross-contamination in the mail.

Anthrax contamination has kept the Hamilton facility closed and forced the closing of a Senate office building for three months. The Senate Hart Office Building, home to half of the 100 senators, reopened Tuesday after it was fumigated.

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