A Justice Department official told a Senate panel yesterday that law enforcement officers might lack the authority to monitor the phone conversations of terrorists and criminals under a proposed law governing calls that travel over the Internet.

Deputy Assistant Attorney General Laura H. Parsky, testifying before the Senate Commerce Committee, said the growing popularity of Internet-based telephone services presents a new threat to law enforcement officials who are already struggling to keep up with an increasingly complex world of wired, wireless and Internet communications.

Parsky testified at a hearing on a Senate bill sponsored by Sen. John E. Sununu (R-N.H.) aimed at ensuring that Internet-based phone services are not subject to the same regulations that govern traditional telephone networks.

Parsky said the deregulatory approach of the bill could undermine the legal authority of law enforcement officials seeking to investigate criminal activity. She also said the Internet-based phone technology could provide a haven for criminals seeking to avoid the kind of surveillance allowed on regular telephone systems.

"While I obviously cannot go into detail on this point, suffice it to say that criminals do not want to be caught, and they are quick to take advantage of any gap in our ability to detect and disrupt their criminal activities," Parsky said.

Parsky's testimony prompted sharp questions from both Democrats and Republicans.

Sen. Ron Wyden (D-Ore.) asked Parsky if investigators had ever been hampered in their efforts to track the Internet communications of criminals. Parsky offered no specific examples.

"You are now looking for a solution for a problem that has not been documented," Wyden said.

Wyden also asked Parsky if companies the provide Internet telephone services are cooperating with federal investigators.

"Some are cooperating, some are not," Parsky said.

Internet-based phone technology allows users to make telephone calls over the Internet. The technology makes it more difficult, but not impossible, for law enforcement officials to listen to conversations.

Internet phone calls are still a tiny percentage of all telephone traffic, but the technology is growing rapidly. The service is available to anyone with high-speed Internet access and is generally cheaper than a regular phone connection. Relatively unknown companies such as Vonage Holdings Corp. helped popularize the technology, but it also is being adopted by industry giants such as AT&T Corp.

The debate over Sununu's bill mirrors tensions between law enforcement officials and Internet phone companies at the Federal Communications Commission. The Justice Department has asked the FCC to require Internet phone companies to design electronic conduits in their networks that would make it easier to tap conversations.

James X. Dempsey, executive director of the Center for Democracy and Technology, said the authority to eavesdrop would not guarantee that law enforcement officials would be able to do it. Technology evolves so quickly that it's virtually impossible for companies to keep up with the detailed demands of law enforcement, Dempsey said.

Dempsey suggested that rather than imposing strict requirements on companies, investigators would be better off working cooperatively with Internet phone providers.