The FBI is investigating 26 unsolved civil rights era cases out of nearly 100 referred to the bureau over the last year, Director Robert Mueller says in calling the protection of civil liberties one of his top priorities.

Mueller was set to testify Wednesday at an FBI oversight hearing before the Senate. Lawmakers were expected to press him about whether his agents violated the civil rights of U.S. citizens whose personal information was obtained secretly in terror and spy investigations.

In a prepared statement sent Tuesday to the Senate, Mueller vows ''to protect the security of our nation while upholding the civil rights guaranteed by the Constitution to every United States citizen.''

''It is not enough to prevent foreign countries from stealing our secrets _ we must prevent that from happening while still upholding the rule of law,'' Mueller says. ''It is not enough to stop the terrorist _ we must stop him while maintaining civil liberties. It is not enough to catch the criminal _ we must catch him while respecting his civil rights.

''The rule of law, civil liberties and civil rights _ these are not our burdens; they are what make us better,'' Mueller says in his written remarks, which were obtained by The Associated Press.

Mueller's remarks offer the first details about the FBI's efforts to reopen decades-old civil rights cases since the successful prosecution last summer of a reputed Ku Klux Klansman for his role in the 1964 abduction and killing of two black teenagers.

Early last year, more than 100 unsolved cases were referred to the FBI. Mueller said 95 of them were sent to investigators in 17 field offices around the country. Ultimately, 52 cases were opened and 26 of those were being reviewed by the Justice Department ''to determine if additional investigation is necessary,'' he said.

''For those cases in which we can move forward, we will,'' he said.

Democrats who control the Senate Judiciary Committee, however, were expected to focus on whether FBI missteps over the last year _ in civil rights and other areas _ have been corrected.

Senate aides for several Democrats said Mueller will probably be asked about the FBI's use of national security letters, which are used under the USA Patriot Act to pursue suspected terrorists and spies.

An audit last year by the Justice Department's inspector general found that FBI agents and lawyers, from 2003 to 2005, demanded personal data on people from banks, telephone and Internet providers, credit bureaus and other businesses without official authorization and in non-emergency circumstances.

The inspector general is expected to issue a follow-up audit at any time that will focus on the FBI's use of national security letters in 2006. Several Justice Department and FBI officials familiar with the upcoming report say it will conclude that the letters were wrongly used at a similar rate as during the previous three years.

But the officials noted that the new audit only examines national security letters that were issued before the FBI was notified of the problems in March 2007 and changed its system. The officials spoke on condition of anonymity because they were not authorized to discuss the audit publicly.

Senate aides said Mueller also probably will be asked about the FBI's failure to pay phone bills on time, prompting telephone companies to cut off wiretaps used to eavesdrop on suspected criminals. In at least one case, a wiretap used in a Foreign Intelligence Surveillance Act investigation ''was halted due to untimely payment,'' according to a January internal Justice audit.

FISA wiretaps are used in the government's most sensitive and secretive criminal and intelligence investigations, and allow eavesdropping on suspected terrorists or spies.

Source: Associated Press