The mailbox was found on Thursday night in Princeton, New Jersey, and has been sent to a U.S. Army facility in Aberdeen, Maryland, for forensic analysis, U.S. Postal Service spokesman Dan Mihalko said.
He said the mailbox was discovered as investigators checked hundreds of boxes from which mail is funneled to a postal sorting center in Trenton, New Jersey, where four anthrax-laced letters were postmarked last year.
"We've been looking at all the mailboxes that feed into the Trenton facility," Mihalko said. "One of them did test positive."
The mailbox was sent to the army laboratory for more conclusive tests, he said, adding, "We've seen in the past, field tests that turn up false positives."
New Jersey Governor James E. McGreevey said there is no danger to the public. He spoke at a press conference with the state attorney general, David Samson, and Louie Allen, special agent in charge of the FBI's Newark office.
"No new cases have been detected beyond the original cases in October 2001," McGreevey said. "With the subsequent removal of the mailbox, no further threat exists."
The mailbox is a standard blue public drop-box that was located on Nassau Street near Princeton University.
Its mail was fed through the sorting facility in Hamilton Township, and was among 600 chosen to be swabbed for anthrax spores by the U.S. attorney's office in Newark, McGreevey said. He did not know how or why the mailboxes were chosen.
Thirty-nine of the 600 mailboxes have yet to be tested.
Dr. Clifton Lacy, commissioner of the state Department of Health and Senior Services, said the test results came back positive Thursday at the department's Public Health and Environmental Laboratory.
Lacy said a department survey of 240,000 emergency room visits and 7,100 stays in intensive care units in and around the state following the October infections turned up no new anthrax cases.
Michael Drewniak, a spokesman for the U.S. attorney's office, declined to say what effect the positive test might have on the investigation.
The FBI launched an investigation in October to find the person responsible for the anthrax attacks. With the trail growing colder 10 months after the attacks, some top researchers are increasingly skeptical investigators will ever find evidence linking anyone to the crime.
Dr. Steven J. Hatfill, a biowarfare expert whose name surfaced more than a month ago, has attracted attention because of intriguing circumstances about his past. He is one of about 30 people being scrutinized.
But law enforcement officials have described Hatfill, 48, as a "person of interest," not a criminal suspect.
Investigators have searched Hatfill's Frederick, Md., apartment twice, including testing for anthrax residue, as well as his car, a storage unit in Florida and his girlfriend's home. They have seized his computer and bags of personal items he had thrown away in preparation for moving. But they have found nothing to link Hatfill - or anyone else - to the tainted letters.
Hamilton handles mail for 46 area post offices and has been closed since Oct. 18. The office processed anthrax-tainted letters sent to NBC anchor Tom Brokaw, U.S. Sens. Tom Daschle and Patrick Leahy and the New York Post.