Wilmer Cutler Pickering Hale and Dorr has been caught up in an email mix-up that revealed secret US Securities and Exchange Commission (SEC) and internal investigations at PepsiCo, after a WilmerHale lawyer accidentally sent a Wall Street Journal (WSJ) reporter privileged documents detailing a history of whistleblower claims at the company.

The internal investigation revolves around PepsiCo's 2011 acquisition of Russian drinks company Wimm-Bill-Dann and the departure of general counsel Maura Smith in 2012 following allegations of financial misreporting and other wrongdoing at PepsiCo. A subsequent SEC investigation into Smith's dismissal, and whether the company fired her in violation of whistleblower laws, is "at an early stage," the WSJ reported.

The WSJ report, SEC Probes Departure of PepsiCo's Former Top Lawyer, outlined how its reporter learned details about the years-old internal investigation started by Smith and about the more recent SEC probe, for which Smith was subpoenaed. The information included a 31 August memo about Smith's subpoena and her contact with federal investigators that was "mistakenly sent by a WilmerHale attorney to a Wall Street Journal reporter as part of communication to other attorneys working on the matter", the report said.

WilmerHale's explanation and apology, sent from a spokesman, came less than three hours after the WSJ published its report. The law firm said it "inadvertently" leaked privileged information by email, then asked the WSJ reporter to delete what he received. WilmerHale accuses the WSJ of going back on its word to delete leaked documents.

In a statement, WilmerHale statement said: "We deeply regret that privileged documents were inadvertently emailed to a reporter at The Wall Street Journal. WilmerHale takes full responsibility, and we apologise to our client. We promptly advised The Wall Street Journal of the error and asked the reporter to delete the material. The reporter told us he had deleted the material, but we later learned he had printed and retained hard copies.

"We are disappointed that the WSJ has decided to publish private information it knew was protected by our client's legal privilege. We are taking additional measures designed to ensure that emails are not misaddressed to unintended recipients."

WSJ reporters Andrew Ackerman, Joe Palazzolo and Jennifer Maloney shared a byline on the report. Ackerman and Palazzolo cover financial regulation and law enforcement for the newspaper, while Maloney writes about the beverage industry.

Gibson Dunn & Crutcher, according to the WSJ report, appears to have conducted the company's internal investigation following a whistleblower tip about Wimm-Bill-Dann. The WSJ said the documents it received indicated that lawyers "found no conclusive evidence of more serious violations" of the Foreign Corrupt Practices Act but did find "potential violations" related to accounting that PepsiCo later addressed.

In additional to WilmerHale, PepsiCo-side lawyers involved in the SEC investigation response include former SEC Commissioner Mary Jo White of Debevoise & Plimpton, the report said.

The documents mistakenly sent to the WSJ include a copy of a 2012 memo Gibson Dunn worked on for Smith that was intended for – but never got to – the PepsiCo board, according to the report. That memo accused company executives in Europe of not vetting the Wimm-Bill-Dann deal correctly. Smith left the company eight days after the memo was drafted "to pursue other opportunities", PepsiCo said at the time.

Less than a week later, the Fortune 50 company announced former deputy attorney general Larry Thompson would become general counsel and head of government affairs, a position he had left a little more than a year before. Tony West, the number three lawyer in the Justice Department during some of the Obama years, has been PepsiCo's general counsel and head of government relations since 2014. Thompson moved to private practice at Finch McCranie, a white-collar boutique in Atlanta, in 2015.

PepsiCo's $3.8bn partial acquisition of Wimm-Bill-Dann brought in legal counsel from Davis Polk & Wardwell in the US alongside Linklaters and Dechert in Russia, according to prior coverage by The American Lawyer. Latham & Watkins advised Wimm-Bill-Dann.

Though it was nonamed in that tie-up, WilmerHale has been in the mix of PepsiCo outside counsel for transactional business previously. A 2006 brochure on WilmerHale's M&A work touted how the firm served as "counsel of choice" on PepsiCo's acquisition of Stacy's Pita Chip Company.

The WSJ report did not name any private practice lawyers working for PepsiCo on the SEC or internal investigations matters, other than White at Debevoise. The report further asserted that the SEC investigation "may not lead to any enforcement action" and attributed that information to people familiar with the documents.

PepsiCo said in its statement to the WSJ that it did not retaliate against Smith, that the company was cooperating with the SEC and found no "material" misconduct at Wimm-Bill-Dunn that would affect the company's financial disclosures.

Smith left PepsiCo with a $10.6m severance package and a non-disparagement agreement, according to an SEC filing. Previously general counsel for a General Electric subsidiary, for building materials company Owens Corning and for International Paper, Smith had been in the position at PepsiCo for 13 months. She now has her own solo law firm in New Canaan, Connecticut, and New York City that provides mediation and arbitration services.

Brian Baxter contributed to this report