Wal-Mart, based in Bentonville, Arkansas, said that last November it voluntarily disclosed potential violations of the Foreign Corrupt Practices Act to the U.S. Justice Department and U.S. Securities and Exchange Commission. The company said that outside law firms are helping the U.S. probe the bribery claims, and it's reviewing its compliance program worldwide.

The case came under scrutiny after The New York Times reported April 21 that Wal-Mart covered up an internal probe begun in 2005 after a company lawyer said he funneled bribes to Mexican officials. On May 17, Wal-Mart said the “ongoing media and governmental interest” could impact the perception of “its role as a corporate citizen.”

On May 22, the California State Teachers' Retirement System, which holds less than 1 percent of Wal-Mart's shares, said it would vote against the entire board and questioned whether directors can be counted upon to oversee the bribery investigation.

“Calstrs does not have confidence the current board has the independence and leadership needed to address these difficult issues,” Calstrs Chief Executive Officer Jack Ehnes said in a statement.

The California Public Employees' Retirement System, which also owns less than 1 percent of outstanding shares, said it plans to vote against the re-election of nine directors, including Walton, Duke and his predecessor Lee Scott.

While the board vote will be largely symbolic because the Walton family controls 47 percent of Wal-Mart shares, the pressure on the company will probably intensify.

Cracking Down

The federal government has been cracking down harder on overseas bribery since the Bush administration, when the Justice Department began aggressively targeting executives.

“Federal prosecutors have said they don't want to use the organization to be a scapegoat,” said Brandon Garrett, a law professor at the University of Virginia. “The last thing the government wants to do is fine the company and see all of the individuals go unpunished.”

The Justice Department has suffered setbacks in its efforts to prosecute individuals. Still, of at least 93 people charged from 2005 to 2012, prosecutors won convictions against at least 47 of them, according to the law firm Shearman & Sterling LLP. The 31 defendants who've been sentenced got an average of 2 years and 2 months in prison.

Prosecutors are charging higher-level executives even if they aren't accused of directly making a bribe, said John Kelly, an attorney with law firm Bass, Berry and Sims who once worked for the Justice Department. If they allowed the bribes to continue, they could be prosecuted, he said.

The difficult part of these cases is proving that executives knew company managers were engaging in illegal activity and failed to take action or looked the other way, according to Kelly.

“You have to have proof of them ignoring the truth and that's difficult,” Kelly said.

Because these investigations move slowly, some Wal-Mart executives could be retired before charges are filed. It took the Justice Department three years from the time it settled a case against Siemens AG to charge individual executives.

Even if Wal-Mart executives aren't charged, the company may have to remove anyone involved to maintain credibility with investors and auditors, said Will Barry, an attorney with Washington law firm Richards Kibbe and Orbe.

“It's possible that external auditors aren't comfortable accepting representations from these people,” Barry said.

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