U.S. officials are warning Americans to be on the lookout for suspicious mail that might carry anthrax. The bacterium has infected four employees of U.S. news organizations, and mail delivery is the suspected route.

U.S. law enforcement, health, and postal officials are advising Americans to be cautious about mail from unknown sources. Attorney General John Ashcroft is urging citizens to let authorities know if suspicious letters and packages arrive.

"If individuals receive mail of which they are suspicious, they should not open it, they should not shake it, they should leave the area of the mail, call the local law enforcement authorities [and] public health authorities so that the mail can be appropriately dealt with," said Mr. Ashcroft.

The attorney general spoke hours after an NBC television network news employee in New York tested positive for anthrax in her skin, the fourth confirmed U.S. case since last week.

The latest case and one in Florida that killed a man developed after the individuals opened suspicious mail. Two other Florida cases are of mailroom employees at a newspaper where the dead man worked. The cases in the two states differ, however, in that the anthrax in Florida was inhaled.

Mr. Ashcroft stresses the government's criminal investigation arm, the Federal Bureau of Investigation, is probing the cases with public health officials to determine if they are connected. "The Federal Bureau of Investigation has launched a separate criminal investigation to find out the source of the anthrax in the New York City case," he said. "No conclusions have been reached at this time. At this time we do not have any evidence that links the anthrax case in Florida to the New York matter. The FBI offices are coordinating their efforts to bring their full investigative resources to bear."

Standing next to Mr. Ashcroft, U.S. Health Secretary Tommy Thompson sought to allay public fears of an anthrax epidemic. "Anthrax is not contagious," he said. "It cannot be transmitted from one individual to another. People should not hoard the antibiotics. We have enough antibiotics to get to the people who need it."

Mr. Thompson says the U.S. public health system is responding aggressively to the anthrax cases. Both he and the attorney general say no evidence links them to terrorism and the attacks against New York and Washington last month. Nevertheless, Mr. Thompson adds that the U.S. public health system is on heightened alert for terrorist biological attacks.