In his most extensive public comments about anthrax, President Bush used his weekly radio broadcast Saturday to try to wrap soothing words and calming facts around a week of mixed signals and escalating public anxiety.

He called the anthrax cases "a second wave of terrorist attacks upon our country" and said he was proud of how law enforcement and postal officials were responding to such an unprecedented biological attack.

"And as we deal with this new threat, we are learning new information every day," Bush said.

On some days, new anthrax information came in by the hour and caught administration officials unaware, forcing the White House to defend itself against questions about whether Bush and his administration were acting aggressively enough. The president left the task of updating the public to his Cabinet secretaries and Homeland Security Director Tom Ridge.

Bush took up that task himself on Saturday with a radio address that read like a public service announcement.

"The government is swiftly testing post offices and other sites for anthrax spores, and is closing them where potential threats to health are detected," he said. "We are working to protect people based on the best information available."

Bush planned three high-profile events next week to reinforce his message of control and command: a Tuesday update on the anti-terror war, delivered via satellite to a gathering in Warsaw, Poland, to leaders of Central European states; a progress report Wednesday on the choke-off of terrorist money; and a prime-time speech outside Washington on Thursday to discuss how he is shoring up America's defenses against further attacks.

On Friday, the nation's 17th case of anthrax was confirmed by the Centers for Disease Control and Prevention and top health officials said they expect more people to fall ill.

The president gave Americans two assurances in his address Saturday — that anthrax is not contagious and that deadly spores in the mail can be killed by sterilization, which the Postal Service is taking steps to do nationwide.

"More than 30 billion pieces of mail have moved through the Postal Service since September the 11th, so we believe the odds of any one piece of mail being tainted are very low. But still, people should take appropriate precautions. Look carefully at your mail before opening it, tell your doctor if you believe you may have been exposed to anthrax," he said.

The president directed listeners to a list of anthrax symptoms on the CDC Web site and reiterated doctors' warnings against taking antibiotics when they're not needed.

"So use antibiotics only after consulting a health care professional. If you see anything suspicious, or have useful information, please contact law enforcement authorities," Bush said.

And, he said, pranksters sending false anthrax alarms should beware: "We will pursue anyone who tries to frighten their fellow Americans in this cruel way."

He promised that the federal government would quickly share with the public all "confirmed and credible" information about the unfolding anthrax threat.