The mail advertising company Advo doubled its share Thursday, from $250,000 to $500,000.
Postal officials, who are still working out the final details, say it will be raised to about $2 million possibly within a week or so.
Originally, a $1 million reward was offered for information leading to the arrest of anyone mailing anthrax. The amount is expected to continue to increase as other government agencies add more.
Between the cost of sanitizing the mail, and the lost business resulting partly from the anthrax scare, the postal service is more than $500 million in the red so far this fiscal year.
Meanwhile, FBI agents examined college photocopying machines last week, looking for links to four anthrax-tainted letters mailed from central New Jersey.
Two agents arrived Friday at the Waksman Institute of Microbiology, a Rutgers affiliate in Piscataway, and asked protein biochemist Richard H. Ebright for the access code needed to operate the photocopiers.
They also checked machines at the College of New Jersey in Ewing, NJ.
"I asked whether it was related to the (anthrax) investigation," he said in a phone interview Tuesday. "The male investigator said, 'We can't leave any stones unturned so we're turning over stones,' or something to that effect."
The agents spent about 10 minutes at each Waksman Institute photocopier, making copies and slipping them into a portfolio or large envelope, Ebright said.
He said agents tested photocopiers in other buildings, but he did not know which ones or how many machines.
"It's obviously the geographic link," Ebright said. "The letters were mailed from this state, not more than 30 miles from this location."
Rutgers spokeswoman Sandra Lanman said Tuesday that it is "the university's policy is not to comment on investigations by outside agencies."
No anthrax research is conducted at Rutgers, Lanman said.
FBI spokeswoman Sherri Evanina would not comment Tuesday on the Rutgers visit or say whether similar tests were conducted elsewhere.
"Of course the investigation is ongoing into who sent the anthrax letters," Evanina said. "We're pursuing all possible leads."
Experts say photocopiers leave subtle clues on paper that can narrow the search for where copying was done. The makeup of ink also can provide clues.
At least four anthrax-laced letters passed through a Trenton-area mail facility. Two, addressed to NBC News anchor Tom Brokaw and the New York Post, were postmarked Sept. 18. Two others were postmarked Oct. 9 and mailed to Sens. Tom Daschle, D-S.D., and Patrick Leahy, D-Vt. All four had block letters and used the date Sept. 11, 2001.
The letters to Brokaw and the Post appeared to be photocopies and the Daschle and Leahy letters also appeared identical.
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