HALIFAX -- The RCMP began investigating a Halifax navy intelligence officer who later pleaded guilty to espionage after the FBI alerted them of a possible security breach, documents made public Thursday say.

Redacted versions of search warrants executed in the case of Sub-Lt. Jeffrey Paul Delisle, 41, were unsealed after the federal Crown consented to their release. The application to obtain the warrants was made by the CBC.

The warrants were used to obtain evidence against Delisle, who pleaded guilty last month to breach of trust and two charges of passing information to a foreign entity that could harm Canada's interests.

One of the documents says the RCMP opened an investigation into Delisle's activities after it received a letter from FBI assistant director Frank Figliuzzi notifying them of a possible security breach involving a Canadian military officer.

That letter was sent Dec. 2, 2011, about six weeks before Delisle was arrested.

"With this information, the RCMP's Integrated National Security Team opened an investigation," one of the documents unsealed Thursday says.

The portions of the documents that were released do not elaborate on how or when the FBI became aware of the security breach. But they do indicate that the RCMP heavily relied on information from Anthony Buckmeier, a Russian counter-espionage specialist who began working for the FBI in 1987.

"Given his vast experience in Russian counter-espionage, I believe the information supported by the opinion of Anthony M. Buckmeier is credible," says a warrant filed by the RCMP.

The documents say the RCMP set up phone taps from Montreal as they pursued their investigation into Delisle's activities. Delisle was arrested Jan. 13.

The documents also say Delisle received a total of 23 money transfers from July 6, 2007, to Aug. 1, 2011 from Moscow and Ireland.

During his bail hearing in March, the provincial court in Halifax heard that Delisle walked into the Russian embassy in Ottawa and offered to sell them information. Over the course of nearly five years, Delisle accepted money transfers from Russia in exchange for his services, the court was told.

There was a publication ban on evidence and arguments presented at the proceedings in the spring, but his guilty plea means there will not be a jury trial now. Sentencing is scheduled for Jan. 10.

The Crown gave a detailed accounting during Delisle's bail hearing of how it alleged he transmitted the information from Trinity, a military intelligence centre in downtown Halifax, to his home and then on to Russian agents.

Delisle would search for Russian references on his work computer, transfer it to a USB key and take it to his home nearby before pasting it into an email program that he shared with his foreign handler, the Crown said.

But at some point, the RCMP hacked into the email account Delisle shared with his Russian handler, the court heard. Delisle continued sending sensitive information through the account, unaware that the Mounties were receiving it.

He came to the attention of Canadian authorities in September 2011 when he returned from a trip he took by himself to Brazil with credit cards found in his luggage, according to the search warrants.

"As mentioned above by the FBI and expert witness Anthony M. Buckmeier, meetings between agents and informations (blacked out) take place in a country foreign to the informant," Canada Border Services Agency says in a report.

"The circumstances of this trip to Brazil lead me to believe Jeffrey Paul Delisle met a (blacked out) agent during his trip."

When he was arrested, Delisle was working as a threat-assessment analyst at Trinity, which provides tactical assessments to Canadian warships and aircraft, both at home and overseas.

The classified information he had access to was shared by the so-called Five Eyes group, which includes Canada, Great Britain, the United States, New Zealand and Australia.

Delisle, who joined the navy as a reservist in 1996, became a member of the regular forces in 2001 and was promoted to an officer rank in 2008. He became the first person in Canada to be convicted under the Security of Information Act, which was passed by the House of Commons after the Sept. 11 attacks.