WASHINGTON (AP) - The FBI and U.S. Postal Service on Thursday offered a reward of up to $1 million for information leading to the arrest of those who sent anthrax through the mail. Investigators continued to link the various incidents through evidence.

"Once again we call upon the public to assist us in this fight against terrorism," FBI director Robert Mueller said 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."

Potter urged citizens to use commonsense caution if they receive suspicious packages, and said his service was mailing postcards to all mail recipients with guidelines. "The best defense that we have right now is an educated American public," he said.

Early tests on a package that was returned from Malaysia to a Microsoft Corp. office in Nevada with a powdery substance came back negative for anthrax at the Centers for Disease Control and Prevention, Homeland Security Director Tom Ridge announced Thursday. Additional tests were being conducted Thursday.

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.

Ashcroft also hailed Thursday's sentencing in New York of four people convicted earlier this year in the 1998 bombing of two American embassies in Africa.

"Today's sentence sends a message," he said. "The United States will hunt terrorists down and will make them pay the price for their evil acts."

The announcements came as authorities pressed to identify the source of the anthrax and to find possible links between the cases that have cropped up in Washington, New York and Florida. Six people were confirmed to have anthrax, and authorities were examining up to three additional cases. Many more people have tested positive for exposure to the bacteria.

Mueller said his agency had received several leads since authorities made public copies of two of the handwritten envelopes in which anthrax was sent. But both he 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 anthrax found at American Media Inc., a tabloid newspaper publisher where one man died from the contamination and another is hospitalized, and anthrax sent to NBC News anchor Tom Brokaw.

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 anthrax found in a letter sent to Sen. Tom Daschle, D-S.D., 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.

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

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.