The Obama administration continued Wednesday to resist the efforts of two Democratic senators to learn more about the government’s interpretation of domestic surveillance law, stating that “it is not reasonably possible” to identify the number of Americans whose communications may have been monitored under the statute.

In a letter to Sens. Ron Wyden (D-Ore.) and Mark Udall (D-Colo.), Kathleen Turner, director of legislative affairs for the Office of the Director of National Intelligence, also said that a joint oversight team “has not found indications of any intentional or willful attempts to violate or circumvent” the Foreign Intelligence Surveillance Act or FISA, which was amended in 2008.

“Every time the American public finds out that laws have been rewritten in secret or the administration can’t give a basic answer, it erodes public confidence and makes it harder for intelligence agencies to do their jobs,” said Wyden, who for the past two years has decried what he calls a de facto “secret law” governing domestic surveillance. He and Udall say they will again offer an amendment to the current intelligence authorization bill to compel the administration to reveal the legal basis for certain intelligence-gathering activities.

Wyden’s concern and the broader issue of excessive government secrecy in the pursuit of national security form the basis of a report to be issued Thursday by the American Civil Liberties Union.

“We are now living in an age of government secrecy run amok,” said the report, written by Mike German, a former FBI undercover agent, and Jay Stanley.

The report notes that during the debate in the spring to reauthorize the Patriot Act, which also amended FISA, Wyden and Udall pressed the administration to disclose the legal rationale for some surveillance measures.

The ACLU report also said that the government is unnecessarily classifying vast amounts of information, undermining efforts to hold officials accountable and facilitate an informed public debate.

The government made a record 76.8 million classification decisions in 2010, according to official statistics cited in the report. The 40 percent rise over the year before is due to the explosion in the use of electronic communications and new requirements to count and report every e-mail, blog entry or other such missives that include classified information, officials said. Problems with classification are common, said Steven Aftergood, director of the Federation of American Scientists’ Project on Government Secrecy. He noted that the National Security Agency, the nation’s largest spy agency, only last November declassified a 200-year-old document on cryptography that had been published and posted online for years. Although the NSA last month hailed the disclosure as evidence of its commitment to meet the requirements of President Obama’s policy on transparency, it turned out that the text, captured by U.S. troops after World War II and placed in intelligence files, was never classified in the first place, Aftergood wrote in a blog posting Wednesday.

The document was available in German libraries, and the full 532-page text of the 1809 study was digitized several years ago and published online through Google Books, Aftergood said.

He called the NSA’s move “a disturbing sign of futility and irrelevance in the nation’s declassification program.”

William A. Cira, acting director of the National Archives’ Information Security Oversight Office, established in 1978 to oversee government classification activities, acknowledged that there are “always going to be some examples of some things that didn’t need to be classified.” But, he said, “what gets classified by and large needs to be classified.”

The ACLU urged Congress to take the lead in challenging practices allowing excessive secrecy. The group recommended expanding the number of lawmakers who should be notified of secret intelligence activities and limiting the types of information that the government may classify. At a hearing Tuesday, a senior NSA official acknowledged to Wyden that the special FISA court, which conducts proceedings in secret, has issued opinions pertaining to key portions of the amended surveillance law but that they could not be discussed in an unclassified setting. In her letter to Wyden and Udall, Turner said the Office of the Director of National Intelligence and the Justice Department are reviewing the opinions to see whether portions can be declassified and released to the public.

In unrelated court proceedings, the government has sometimes tried to make substitutions for classified information, as happened recently in a failed bid to win a felony conviction for a former NSA official suspected of mishandling classified information.

When the trial judge demurred, the prosecution withheld certain evidence rather than put forward what it deemed to be sensitive data. Among the terms the government sought to keep secret was a phrase describing a certain technology. According to a person familiar with the case, the term was “fiber optic.”