U.S. government officials believe the Sept. 11 terrorism network's plotting against the United States is far from over.

They are worried that more attacks on U.S. interests, either at home or abroad, may take place if and when the United States and its allies retaliate for the attacks on New York and Washington, sources tell Fox News.

Such dire warnings are coming from other circles as well.

At a classified briefing earlier this week, in response to a senator's question about the probability of a new terrorism threat, one intelligence official said there is a "100 percent" chance of an attack should the United States strike Afghanistan, sources familiar with the briefing told The Washington Post.

Based on new and credible information, U.S. intelligence officials told members of Congress that it is very likely that terrorists associated with alleged mastermind Usama bin Laden will attempt another major attack on American interests in the near future, The Post reported.

"The threat remains in the U.S.," said White House Ari Fleischer Friday. "I can't quantify that threat. The president has been forthright with the American people. That is why the president is so determined to take action against terrorists and those who harbor them."

If other attacks were planned, the terrorists have gone to great lengths to conceal them. Officials said the conspirators communicated frequently via the Internet and e-mail, often disguising their plans using code language or pictures that made them appear to be innocent communications.

The terrorists frequently accessed the Internet in public places like libraries and hotels, making it difficult to track their whereabouts or identities. Several Internet providers in the U.S. and abroad have received subpoenas to turn over information about the communications.

Alexis Debat, a former French Defense Ministry official, said that one man arrested in connection with a plot against the U.S. Embassy in France had a code book for reading disguised messages. The suspect is believed to be a part of bin Laden's terrorist network, Al Qaeda.

Debat also said French officials believe terrorists would have received their final instructions for the plot as a hidden message on the Internet.

According to Marc Enger, former director of operations for U.S. Air Force intelligence, some U.S. agents have reported bin Laden has used steganography — the art of hiding a message in plain sight.

"These messages can be hidden in e-mail or in a downloaded picture," said Chet Hosmer, president and CEO of WetStone Technologies, which has been one of the companies providing the FBI with the means to detect the hidden messages.

What investigators have been able to glean from the Web surfing logs they could find has alarmed them. And in Canton, Ohio, a man who said he was Egyptian went to a library there over the summer and asked for detailed maps of the city's water system as well as books about microbiology, parasitology and diseases that can be passed from animals to people, a librarian who helped him told the Akron Beacon Journal.

The unidentified man is associated with two former Canton men arrested in Detroit last month in connection with the probe into the Sept. 11 attacks.

President Bush said U.S. and overseas authorities have 150 people in custody thought to be part of bin Laden's Al Qaeda. The fugitive Saudi multimillionaire is the prime suspect in the attacks, in which hijackers crashed commercial jetliners in New York, Washington and southwestern Pennsylvania, killing more than 6,000 people.

The British government said Thursday that at least three of the 19 suspected hijackers had links to Al Qaeda and one had played a key role in the bombings of U.S. embassies in Africa and the USS Cole. Furthermore, one of bin Laden's top lieutenants has admitted Al Qaeda's involvement in the Sept. 11 attacks, British Prime Minister Tony Blair said.

Bin Laden was indicted in the United States for the embassy bombings and is suspected of planning the attack on the Cole in Aden, Yemen.

The British report released by Blair did not name the hijackers. Two suspected hijackers, Khalid Almihdhar and Nawaf Alhazmi, met with a bin Laden associate believed to be involved in the Cole attack in January 2000 in Malaysia, law enforcement officials have said.

The British report also said the senior bin Laden lieutenant acknowledged he oversaw the planning of the Sept. 11 attacks. U.S. officials say Mohamed Atta, suspected of flying one of the hijacked planes that crashed in New York, wired money to Shayk Saiid, believed to be bin Laden's top money man.

In other developments:

—The FBI in Maine released photos of Atta and Abdulaziz Alomari taken from surveillance videos at an ATM, a gas station and a Wal-Mart just hours before they boarded a commuter flight in Portland. Their flight linked up with one of the jetliners that crashed into the World Trade Center.

— A man suspected of being involved in the plot was arrested in Mauritania. Mouhamedou Ould Slahi had been under surveillance in Canada for suspected links to foiled millennium attacks.

The Associated Press contributed to this report.