By Paritosh Bansal and Jason Lange

NEW YORK/WASHINGTON, July 31 (Reuters) - A U.S. Securities and Exchange Commission watchdog conducted an extensive, months-long investigation to find out who had leaked information that appeared in two Reuters stories published last autumn, but it was unable to identify the sources, according to a report of the probe seen by Reuters on Thursday.

The SEC's Office of Inspector General (OIG) started the investigation after Reuters published information about the regulator's decision, taken in a closed-door meeting on Sept. 12, 2013, to settle its probe into JPMorgan Chase & Co's massive London Whale trading loss.

The investigation started after Reuters reported on Sept. 17, 2013 that the regulator had approved its portion of a $700 million civil settlement with JPMorgan in a split vote. The story also said that two of the five commissioners had recused themselves from the decision.

The SEC made its decision public on Sept. 19, 2013. About a week later, Reuters reported further non-public details about why one of the commissioners, Michael Piwowar, had voted against the settlement.

In its report, dated March 5, the watchdog said it interviewed as many as 53 SEC employees, including Chair Mary Jo White, SEC commissioners and other top officials, during the investigation.

An OIG official said its inspectors don't consider issues of press freedom when carrying out their investigations, and that the probe was launched at White's request. "Our office doesn't really differentiate where the information went," said Raphael Kozolchyk, the watchdog's legislative and public affairs counsel.

The SEC declined to comment on the OIG's probe and report. CNBC earlier reported on its contents.

Piwowar raised concerns about the leaked information with White, according to the OIG report. White and Piwowar could not immediately be reached for comment.

Reuters Editor-in-Chief Stephen Adler said: "It's the job of journalists to report vigorously on the workings of government, and to provide readers with news that some may prefer to keep secret. It is in that spirit that Reuters covers the SEC and other agencies of the U.S. government, and we plan to continue to do so while protecting the confidentiality of our sources."

The investigation is the latest example of the Obama administration's aggressive moves to try to plug leaks of non-public information to the media. Last year, the Justice Department was criticized for examining reporter emails and phone records without first notifying the press outlets. Attorney General Eric Holder later met with news media executives and agreed to revise policies for dealing with the press in national security cases.