OTTAWA - An investigation into the so-called NAFTA-gate scandal has exonerated the prime minister's chief of staff and the Canadian ambassador in Washington.

But the 21-page document lays blame at the Foreign Affairs Department for incorrectly classifying a diplomatic document and distributing it to too many people.

Two private investigators hired by Ottawa interviewed 36 bureaucrats, diplomats and others, but could not finger the person who handed the key diplomatic report over to the Associated Press.

In a brief written statement, Prime Minister Stephen Harper highlighted the findings and said he has "accepted all the recommendations of the report."

The recommendations include tighter document security at Foreign Affairs.

The leak, originating from the Canadian Consulate in Chicago, set off a chain reaction which some believe resulted in Democratic Senator Barack Obama's loss in March's Ohio primary.

"The investigation has been unable to determine who leaked the report, to whom it was leaked or whether there was only one leak," the investigators wrote.

"The original diplomatic report was incorrectly classified and had an inappropriately broad distribution list."

The diplomatic report from the consulate in Chicago outlined a meeting between an Obama adviser and the consul general. It was issued by a Mr. de Mora, consul for political and economic relations and public affairs, on Feb. 13.

It quoted a high-level Canadian official as saying comments from Obama and Hillary Clinton about renegotiating NAFTA if either became president was nothing more than campaign rhetoric.

The note was distributed to 232 e-mail addresses, 212 of which were at Foreign Affairs. Eight were outside government.

Friday's report said there is no evidence that the prime minister's departing chief of staff, Ian Brodie, disclosed any classified information when he talked to CTV reporters about the candidates' NAFTA remarks.

And it says there is no evidence that Canada's ambassador to the United States, Michael Wilson, revealed any classified information, information tied to the diplomatic note, or about "any U.S. presidential candidate's position with respect to NAFTA."

Wilson's comments in a telephone discussion with a CTV reporter, however, "likely helped lead the reporter to the Senator Obama campaign."

The investigators reviewed logs of telephone calls placed by "officials of interest" from their government office and cellular phones.

They also looked at transmission logs from fax machines used by those same officials.

And they studied related e-mails to determine "who had knowledge of the information at what point in time, the extent of that knowledge, and whether any inappropriate transmission of information had occurred."