Federal authorities have asked the U.S. Postal Service to distribute an informational flyer about the case to more than 500,000 people in New Jersey and Pennsylvania.
The bureau also doubled the reward Wednesday to $2.5 million for information leading to the capture of whoever sent the anthrax-tainted envelopes that terrorized the nation last year.
The flyers show pictures of the envelopes - each pre-stamped and smaller than business size. The FBI hopes someone will recognize the handwriting or the type of embossed, 34-cent envelope.
"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."
Authorities declined Wednesday to reveal details about the probe.
The FBI is initially targeting the flyers to mail customers in the Trenton area because they suspect the culprit is comfortable there "due to present or prior association" with the area.
The flyers also suggest that the suspect or suspects likely have a scientific background or work history, or "a specific familiarity with anthrax."
The tainted letters were postmarked in Trenton in September and October and went through the Hamilton mail facility. Officials believe several people were infected through cross-contamination in the mail.
Five people have died of anthrax since early October - including two Washington postal workers - and more than a dozen people were infected in New York, New Jersey, Florida and Washington.
In December, the FBI sent out similar fliers to New Jersey and Philadelphia residents in an attempt to jog memories and gather clues about the identity of the anthrax mailer.
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, said Kevin Burke of the Postal Inspection Service. More than 540 post offices have been closed as a result and 71 people have been arrested, Donovan said.
Investigators said earlier this week that some scientific clues appeared promising. Scientists hope identifying genetic markers will allow them to trace the anthrax used in the attacks to one of about a dozen labs that had samples of the commonly held Ames strain.
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