WASHINGTON - The number of Americans being secretly wiretapped or having their financial and other records reviewed by the government has continued to increase as officials aggressively use powers approved after the Sept. 11, 2001, attacks. But the number of terrorism prosecutions ending up in court -- one measure of the effectiveness of such sleuthing -- has continued to decline, in some cases precipitously.

The trends, visible in new government data and a private analysis of Justice Department records, are worrisome to civil liberties groups and some legal scholars. They say it is further evidence that the government has compromised the privacy rights of ordinary citizens without much to show for it.

The emphasis on spy programs is also starting to give pause to some members of Congress who fear the government is investing too much in anti-terror programs at the expense of traditional crime-fighting.

These concerns come as the Bush administration has been seeking to expand its ability to gather intelligence without prior court approval. It has asked Congress for amendments to the 1978 Foreign Intelligence Surveillance Act to make it clear that eavesdropping on foreign telecommunications signals routed through the U.S. does not require a warrant.

Law enforcement officials say the additional surveillance powers have been critically important in ways the public does not always see. Threats can be mitigated, they say, by deporting suspicious individuals or letting them know that authorities are watching them.

"The fact that the prosecutions are down doesn't mean that the utility of these investigations is down. It suggests that these investigations may be leading to other forms of prevention and protection," said Thomas Newcomb, a former Bush White House national security adviser.