NEW YORK (CNN/Money) - The Securities and Exchange Commission is looking into whether six large companies manipulated their assumptions about pension fund performance to affect their earnings, according to a published report.

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Business Week reports that the SEC Enforcement Division sent letters to six companies earlier this month requesting documents, including e-mails between those who set assumptions, explaining how they arrived at their estimates. The article did not identify the six companies.

SEC officials told the magazine it had no evidence yet of wrongdoing, but they are concerned.

SEC Enforcement Director Stephen Cutler told Business Week his department is "looking to see whether managers are adjusting their assumptions with an eye to enhancing the earnings and balance-sheet numbers investors care about. That could be fraud."

Some companies assume returns of 9 percent or more on pension fund assets, which the magazine said experts and SEC officials believe is tough to justify.

The SEC Corporation Finance Division has been making general warnings for some time about using what it considered overly rosy pension assumptions, saying the regulators might challenge rate-of-return assumptions above 9 percent.

The magazine said companies report their projected earnings for their pension plans, not actual returns. It reported that if the assumed return on pension investments exceeds the pension cost, the company can count the surplus in its earnings even if real returns show a loss.