WASHINGTON (AP) - A total of five people have been infected with anthrax, out of "thousands and thousands" that have been tested so far in the scares that have swept the nation, and a sixth possible infection is being evaluated, Homeland Security Director Tom Ridge said Thursday.

"There is a great deal of speculation out there, obvious concern ... to all Americans," Ridge said. "Instead of speculating, we'd like to focus on the facts," said Ridge, in his first news briefing since taking the post last week.

Ridge and other top Bush administration officials sought to calm public anxiety over the anthrax attacks.

FBI Director Robert Mueller said that the government was offering a $1 million reward to information leading to the conviction of anyone responsible for the anthrax attacks

And Postmaster General John Potter announced that the Postal Service would mail within the next week "a postcard to everyone in America" outlining what they should be suspicious of in mail they receive.

"We believe the mail is safe it's very safe if you follow the prudent directions" we are providing, Potter said.

Attorney General John Ashcroft, meanwhile, pledged to prosecute to the "fullest extent of the law" all cases of hoaxes involving anthrax or other biomedical threats.

Four people have been charged in four false anthrax threat cases so far, Ashcroft said.

Such hoaxes "create illegitimate alarm in a time of legitimate concern," Ashcroft said.

Ridge said that so far, "thousands and thousands of people have been tested. Only five people have tested positive at this time for anthrax. We're in the process of confirming a sixth."

He spoke as a new infection of the skin form of anthrax was reported in an assistant to CBS News anchor Dan Rather.

Ridge mentioned the five previously confirmed cases of anthrax infection: two in Florida, an NBC assistant to Tom Brokaw, the infant son of an ABC producer and the new CBS case. He cited a sixth possible infection, but did not elaborate.

In Washington, 31 people in the Senate have tested positive for exposure after an anthrax-laden letter was received in the office of Senate Majority Leader Tom Daschle, authorities said. Deputy Surgeon General Ken Moritsugu said that the 31 were among a total of 3,000 nasal swabs that had been evaluated.

He said that the area of exposure had been on the fifth and sixth floors of the Senate Hart Office Building "and it was confined to individuals who either worked in or traveled through that area on Monday."

"There is continued analysis and evaluation of that area as well as other areas of the Senate and the Capitol and we will be reporting as we get additional information," Mortisugu said.

Tests performed on some 1,200 people at the Capitol complex were likely to show at least a few more were exposed to anthrax, beyond the 31 confirmed, said Sen. Bill Frist, R-Tenn., a doctor. He expressed confidence that early treatment would succeed against any who might end up infected.

In New York, CBS said a woman who works for anchorman Dan Rather tested positive for anthrax. The network said the woman was exposed on the skin, not through inhalation, and is being treated. Skin exposure is less serious than inhalation.