Government officials on Thursday put a bounty on the heads of anthrax mailers: up to $1 million. 

"Once again we call upon the public to assist us in this fight against terrorism," said FBI Director Robert Mueller in a joint announcement with Postmaster General Jack Potter. 

Mueller said the reward would be for "information leading to the arrest and conviction for terrorist acts of mailing anthrax." 

"The best defense that we have right now is an educated American public," said Potter, urging to use common sense with suspicious packages. 

Potter also said the U.S. Postal Service was sending postcards with guidelines to all mail recipients. 

Investigators continued to sift through evidence in the anthrax mailings, which have involved three states and Washington, D.C. 

An assistant to CBS Evening News anchorman Dan Rather in New York City was found to have an anthrax skin infection Thursday morning, and two postal workers in Trenton, N.J., weres found to have the same condition later in the day, raising the number of infected persons to seven. 

An NBC News employee and the child of an ABC News employee had already been diagnosed with skin infections in New York, and a Florida man remained hospitalized with an anthrax lung infection after a co-worker died of the same condition. Thirty-one people on Capitol Hill and several in New York were being treated with antibiotics after having been exposed to, but not infected by, the dangerous bacterium. 

One scare in a fourth state turned out to be a false alarm. Homeland Security Director Tom Ridge said Thursday that a package returned to a Microsoft Corp. office in Nevada from Malaysia with a powdery substance was tested for anthrax by the Centers for Disease Control and Prevention and found to be negative. 

"However, additional samples are going to be taken to ensure there is no threat," Ridge said. 

Authorities announced that four people had been charged with federal felonies for anthrax hoaxes. "We intend to prosecute these hoaxes to the fullest extent of the law," Attorney General John Ashcroft said. 

Ashcroft said among those who have been charged was a man who sent talcum powder through the mail, saying it was anthrax. William Sylvia, of Portsmouth, R.I., was charged with mailing a threatening communication. 

The announcements came as authorities pressed to identify the source of the anthrax and to find possible links between the cases. 

Mueller said his agency had received several leads since authorities made public copies of the anthrax-laden handwritten envelopes mailed to Senate Majority Leader Tom Daschle, D-S.D., and NBC Nightly News anchorman Tom Brokaw. 

Both Mueller and Ashcroft declined to say if there were specific leads that were promising. "We are not in a position to determine those who are responsible," the director said. 

In New Jersey, FBI agents questioned at least one pharmacist about anyone buying 60 to 120 tablets of the antibiotic Cipro, used for treating anthrax, prior to Sept. 18. 

"Anyone trying to buy that many would stick out like a sore thumb," said pharmacist John Berkenkopf, who was questioned. 

A preliminary match was made between the anthrax found at American Media Inc., the Boca Raton, Fla., tabloid newspaper publisher employing the two men with lung infections, and the anthrax sent to Brokaw which infected his assistant. 

Officials at the federal Centers for Disease Control and Prevention said the strain — one of hundreds of varieties of anthrax — occurs naturally and is found domestically in hoofed animals such as cows and deer. Further tests must be done to determine if the strains came from the same source. 

The letter to Brokaw was postmarked Sept. 16 from Trenton, N.J. Investigators believe the Florida man who died of anthrax may have contracted the disease from a letter that was destroyed before he became sick. His last day at work was Sept. 26. 

Still unknown is whether the anthrax sent to Daschle is the same strain as the Florida and New York material. 

The anthrax found in Daschle's office was professionally made, meaning it was manipulated and possibly refined with additives to keep the particle size small enough so that it's more likely to waft and be inhaled, said a federal terrorism expert, speaking on condition of anonymity. Inhaled anthrax is the most deadly form of the disease. 

There's no evidence so far that the anthrax is associated with a weapons program. Iraq and Russia are both believed to have experimented with anthrax as a weapon. 

And no evidence has turned up linking the anthrax attacks to foreign terrorists. One official said some evidence might suggest a domestic source. 

Meantime, House Minority Leader Richard Gephardt, D-Mo., said tests were being conducted on a suspicious letter sent to the office of House Speaker Dennis Hastert, R-Ill. The letter looked like the letters sent to Daschle and Brokaw, Gephardt said, quoting Hastert's staffers. 

"We don't know yet whether it was another anthrax letter. I'm sure we'll find out later in the week," he said Thursday on NBC's Today show. 

Ashcroft said the anthrax attacks could be the work of a group and individuals working independently. 

"It may be that there is some of both here," Ashcroft said on PBS' The NewsHour With Jim Lehrer. 

He said those responsible for mailing anthrax may also be attempting to divert the attention of investigators by perpetrating anthrax hoaxes. 

"There may in fact be some linkage," Ashcroft said. 

Detectives in New Jersey looking for the source of anthrax-contaminated letters sent to NBC and Daschle are using the pre-stamped envelopes and bar codes to narrow their search. The bar codes give a date and approximate time the letters were processed. Postal Inspector Tony Esposito said other information from the bar codes eliminated many of the post offices from the search. Authorities were reviewing post office surveillance tapes. 

A chilling note warning, "You've been exposed to anthrax. You're going to die," was in the letter sent to Daschle's office, said Sen. John Kerry, D-Mass. 

The Associated Press contributed to this report