Anthrax investigation turns to rapid transit card

NEW YORK (CNN) -- Investigators in New York have turned to Kathy Nguyen's MetroCard -- a prepaid ticket that records the time and place of a passenger's use of the rapid transit system -- to help them unravel the mystery of how the hospital stockroom employee came in contact with the anthrax bacteria that killed her.

Nguyen, who was buried Monday, was the only person to have contracted inhalation anthrax with no apparent link to any known spore contamination.

Nguyen's death has been ruled a homicide, but investigators have yet to uncover solid evidence to indicate where she may have picked up the bacteria. They hope the record of her travels during the last two weeks of her life -- recorded on the MetroCard -- can offer a much needed clue.

Authorities believe the three other people who died from the disease -- two postal workers and an employee of a Florida tabloid newspaper -- inhaled spores that had come through the mail system.

Authorities have identified three tainted letters -- one to a Senate office, another to NBC News and a third to the New York Post -- and believe most of those who fell ill, including postal workers, came into contact with those letters. Investigators, however, have not ruled out the possibility of other letters or sources of contamination.

On Tuesday, Democratic senators at a hearing on bioterrorism criticized the FBI's investigation into the anthrax outbreak when the bureau's deputy assistant director for counterterrorism admitted little progress in the cases.

Tim Caruso acknowledged to the Senate panelists that his answers were "unsatisfactory," adding that they were "unsatisfying to us as well."

The three contaminated letters were all postmarked in Trenton, New Jersey, at a regional processing center in Hamilton Township. But investigators have made little progress beyond that facility.

Since the outbreak of anthrax cases began last month, 17 people have been diagnosed with the disease -- ten people with the inhalation variety, including the four who died, and seven with cutaneous (skin) anthrax. Five other people are suspected to have the disease.

The last confirmed case of the disease was eight days ago, leading many officials to surmise that the current round of disease has run its course.