WASHINGTON-The terrorist attacks of Sept. 11, 2001, and threats that followed have challenged law enforcement and intelligence agents in the Bush administration to keep tabs on suspects who rely on prepaid mobile phones and other nontraditional means of wireless access to remain stealth and anonymous.

Indeed, Swiss authorities reportedly have evidence that senior al-Qaeda members tied to the deadly strikes against the United States used mobile-phone cards bought in Switzerland and made calls in Afghanistan and Pakistan. The case involves the sale of Subscriber Identity Module cards, which plug into GSM mobile phones and give consumers fixed amounts of calling minutes. In Switzerland, no identification is required for the purchase.

It is a vexing problem for security officials here and overseas, given that terrorists are highly mobile, tech-savvy and experts at hiding their identity. That is why prepaid wireless phones are so attractive to terrorists. But the problem is not limited to prepaid cell phones or even wireless technology for that matter. Disposable phones can make electronic surveillance difficult. But so can ordinary prepaid long-distance cards and Web-based services such as e-mail, chat rooms and message boards. No wonder then that the FBI's presence at the Federal Communications Commission has grown in recent years.

Since the mid-1990s, law enforcement has butted heads with the cellular industry over implementation of the 1994 Communications Assistance for Law Enforcement Act. More recently, the FBI has shifted its wiretap focus to packet data, broadband networks and voice over IP.

While terrorists are believed to depend on a host of technologies, wireless appears to be their technology of choice. One of the first things CIA and Pakistani agents did after arresting Khalid Shaikh Mohammed, the No. 3 al-Qaeda operative who is suspected of orchestrating the Sept. 11 attacks, was to check his cell phone for phone numbers.

The USA Patriot Act of 2001 strengthened law enforcement's ability to carry out wiretaps, but it did not overcome all the hurdles in electronic surveillance. Worse, it created controversy for the Justice Department and FBI. Changes to the 1978 Foreign Intelligence Surveillance Act, which now allows greater information sharing among federal agencies, have prompted loud protests by civil liberty groups and lawsuits. CALEA is applicable to domestic criminal investigations, while FISA governs wiretaps of suspected foreign agents. Critics allege the Patriot Act has blurred the lines between the two laws.

While much post-9/11 attention has been focused on how terrorists could disrupt computer backbones of critical infrastructure-telecom networks, electrical grids, water utilities and railroads-U.S. officials are far more concerned with how terrorists talk to each other.

"To date, terrorists have posed only low-level cyber threats, but some organizations are increasingly using information technology for communication," FBI Director Robert Mueller told a House appropriations panel last Thursday.

A spokesman for Pakistan's Interior Ministry was quoted recently as saying a new cyber crime division "will play a key role in the days to come in tracing those terrorists who often use the Internet or prepaid telephone cards to communicate messages."

How are U.S. officials responding to surveillance obstacles posed by pay-as-you-go mobile phones, prepaid cards and other modern packaging of wireless access that gives terrorists and criminals virtually anonymity?

In some cases, industry support is critical.

Howard Segermark, head of the International Prepaid Communications Association, has worked with law enforcement since the 2001 terrorist attacks to educate agents. "We can help law enforcement if they don't know the name of a prepaid company," said Segermark. From there, law enforcement can acquire call detail records.

Such call information helped law enforcement link Timothy McVeigh to the Oklahoma City bombing. It is unclear what privacy rights are afforded prepaid callers.

But there are limitations. For example, law enforcement has to know a suspect is using a prepaid card or a prepaid mobile phone.

Some see prepaid wireless and prepaid cards as symptoms of the larger inherent problem of running down well-trained criminals and terrorists, whose business is to avoid detection.

"There are astute criminals who will use technology to mask their identity. It goes beyond prepaid," said Glenn Manishin, a lawyer who represents the International Prepaid Communications Association.

Michael Altschul, general counsel at the Cellular Telecommunications & Internet Association, agreed. "This is an issue that came up before 9/11. Prepaid is not an issue unique to wireless."

As important a tool as wiretaps are to law enforcement, surveillance still requires agents on the ground and human intelligence.

Even Mueller, whose FBI is attempting to transform itself with new technological tools to head off terrorism, conceded there are limits to technology. "I don't believe technology answers all questions, it is not the be-all," the FBI director told lawmakers.