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 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 Bush administration has been seeking to expand its ability to gather intelligence without prior court approval. The [Justice] department ... reported a sharp rise in the use of national security letters by the FBI -- from 9,254 in 2005 to 12,583 in 2006, the latest data available. The letters seek customer information from banks, Internet providers and phone companies. They have caused a stir because consumers do not have a right to know that their information is being disclosed and the letters are issued without court oversight. Civil liberties groups say the new data reveal a disturbing consequence of the government's post-Sept. 11 expanded surveillance capabilities. The number of Americans being investigated dwarfs any legitimate number of actual terrorism prosecutions, and that is extremely troubling," said Lisa Graves, deputy director of the Center for National Security Studies, a Washington-based civil liberties group.