WASHINGTON – An odd-looking Canadian coin with a bright red flower was the culprit behind the U.S. Defence Department's false espionage warning earlier this year, the Associated Press has learned.

The odd-looking – but harmless – "poppy coin" was so unfamiliar to suspicious U.S. Army contractors travelling in Canada that they filed confidential espionage accounts about them. The worried contractors described the coins as "anomalous" and "filled with something man-made that looked like nano-technology," according to once-classified U.S. government reports and e-mails obtained by the AP.

The silver-coloured 25-cent piece features the red image of a poppy – Canada's flower of remembrance – inlaid over a maple leaf. The unorthodox quarter is identical to the coins pictured and described as suspicious in the contractors' accounts.

The supposed nano-technology actually was a conventional protective coating the Royal Canadian Mint applied to prevent the poppy's red color from rubbing off. The mint produced nearly 30 million such quarters in 2004 commemorating Canada's 117,000 war dead.

"It did not appear to be electronic (analog) in nature or have a power source," wrote one U.S. contractor, who discovered the coin in the cup holder of a rental car. "Under high power microscope, it appeared to be complex consisting of several layers of clear, but different material, with a wire like mesh suspended on top."

The confidential accounts led to a sensational warning from the Defence Security Service, an agency of the Defence Department, that mysterious coins with radio frequency transmitters were found planted on U.S. contractors with classified security clearances on at least three separate occasions between October 2005 and January 2006 as the contractors travelled through Canada.

One contractor believed someone had placed two of the quarters in an outer coat pocket after the contractor had emptied the pocket hours earlier. "Coat pockets were empty that morning and I was keeping all of my coins in a plastic bag in my inner coat pocket," the contractor wrote.

But the Defence Department subsequently acknowledged that it could never substantiate the espionage alarm that it had put out and launched the internal review that turned up the true nature of the mysterious coin.

Meanwhile, in Canada, senior intelligence officials expressed annoyance with the American spy-coin warnings as they tried to learn more about the oddball claims.

"That story about Canadians planting coins in the pockets of defence contractors will not go away," Luc Portelance, now deputy director for the Canadian Security Intelligence Service, wrote in a January e-mail to a subordinate. "Could someone tell me more? Where do we stand and what's the story on this?"

Others in Canada's spy service also were searching for answers. "We would be very interested in any more detail you may have on the validity of the comment related to the use of Canadian coins in this manner," another intelligence official wrote in an e-mail. "If it is accurate, are they talking industrial or state espionage? If the latter, who?" The identity of the e-mail's recipient was censored.

Intelligence and technology experts were flabbergasted over the warning when it was first publicized earlier this year. The warning suggested that such transmitters could be used surreptitiously to track the movements of people carrying the coins.

"I thought the whole thing was preposterous, to think you could tag an individual with a coin and think they wouldn't give it away or spend it," said H. Keith Melton, a leading intelligence historian.

But Melton said the Army contractors properly reported their suspicions. "You want contractors or any government personnel to report anything suspicious," he said. "You can't have the potential target evaluating whether this was an organized attack or a fluke."

The Defence Security Service disavowed its warning about spy coins after an international furor, but until now it has never disclosed the details behind the embarrassing episode. The U.S. said it never substantiated the contractors' claims and performed an internal review to determine how the false information was included in a 29-page published report about espionage concerns.

The Defence Security Service never examined the suspicious coins, spokeswoman Cindy McGovern said. "We know where we made the mistake," she said. "The information wasn't properly vetted. While these coins aroused suspicion, there ultimately was nothing there."

A numismatist consulted by the AP, Dennis Pike of Canadian Coin & Currency near Toronto, quickly matched a grainy image and physical descriptions of the suspect coins in the contractors' confidential accounts to the 25-cent poppy piece.

"It's not uncommon at all," Pike said. He added that the coin's protective coating glows peculiarly under ultraviolet light. "That may have been a little bit suspicious," he said.

Some of the U.S. documents the AP obtained were classified "Secret/Noforn," meaning they were never supposed to be viewed by foreigners, even America's closest allies. The government censored parts of the files, citing national security reasons, before turning over copies under the U.S. Freedom of Information Act.

Nothing in the documents – except the reference to nanotechnology – explained how the contractors' accounts evolved into a full-blown warning about spy coins with radio frequency transmitters. Many passages were censored, including the names of contractors and details about where they worked and their projects.

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But there were indications the accounts should have been taken lightly. Next to one blacked-out sentence was this warning: "This has not been confirmed as of yet.''

The Canadian intelligence documents, which also were censored, were turned over to the AP for $5 under that country's Access to Information Act. Canada cited rules for protecting against subversive or hostile activities to explain why it censored the papers.

Associated Press writer Beth Duff-Brown contributed to this story from Toronto.