Critics are urging the Harper government to lift the veil that shrouds Canada's electronic eavesdropping agency after an overseer's report suggested Canadians may have been illegally spied on.

In his final report to Parliament, commissioner Robert Decary says some of the spying at Communications Security Establishment Canada may have affected Canadians in the past year.

However, thanks to poor record-keeping, Mr. Decary – a retired judge who has been the agency's independent watchdog since 2010 – said he can't be sure.

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"A number of CSEC records relating to these activities were unclear or incomplete," said the report, tabled on Wednesday. "After in-depth and lengthy review, I was unable to reach a definitive conclusion about compliance or non-compliance with the law."

CSEC says it did not break the rules. "The commissioner's statement about a lack of records is a reference to a single review of a small number of records gathered in the early 2000s, in relation to activities directed at a remote foreign location," the agency said in an e-mail. "This conclusion does not indicate that CSEC has acted unlawfully. It indicates that certain material upon which the commissioner would have relied for his assessment was incomplete or not available for a number of reasons."

The report comes amid startling revelations about the interception of Internet communications from Americans and British citizens by their respective governments.

But at least one analyst likens Mr. Decary's findings to "accounting errors" amid massive amounts of electronic data.

Canadians need not be alarmed by the report's findings, said Ray Boisvert, until last year a deputy director of the Canadian Security Intelligence Service.

Inadvertently tracking a few innocent Canadians is inevitable, considering the piles of information being sifted through government computers, said Mr. Boisvert, who spent 30 years with CSIS and the RCMP.

"Given the massive volume of millions and billions of pieces of data tracked, those are kind of like accounting errors," he said. "Is there a grand conspiracy to dupe Canadians or to pry [into their personal lives]? No, absolutely not."

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The report nonetheless contradicts statements made in June by Peter MacKay, the defence minister at the time, denying the government was spying on Canadians, NDP defence critic Jack Harris said Thursday.

Any illegal snooping on Canadian citizens "has to stop," Mr. Harris said.

"Minister of National Defence Rob Nicholson needs to release all information related to this spying immediately," he said, noting that the resumption of Parliament is being put off until at least October.

Julie Di Mambro, a spokeswoman for Mr. Nicholson, said the government takes the privacy of its citizens seriously, and that targeting of Canadians by the spy agency is not tolerated.

"The privacy of Canadians is of utmost importance," she said in an e-mail. "CSEC is prohibited by law from directing its activities at Canadians anywhere in the world or at any person in Canada."

The end game for any spy agency is to prevent attacks, such as the Boston Marathon bombing in the United States, Mr. Boisvert said. As such, striking a balance between protecting people and their privacy can be tricky.

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CSEC is forbidden from intentionally collecting or analyzing information from Canadian citizens in Canada or abroad. However, the National Defence Act allows the defence minister to give CSEC written ministerial authorization to intercept private communications unintentionally while collecting foreign-signals intelligence.

Mr. Decary, who is stepping down for personal reasons, has agreed to stay on for another three months until a successor can be appointed.

But on his way out the door, the commissioner is critical of the Harper government, accusing it of taking too long to make changes to anti-terrorism provisions of the defence act.