The Securities and Exchange Commission is investigating whether bond giant Pacific Investment Management Co. artificially boosted the returns of a popular fund aimed at small investors, according to people familiar with the matter, the latest challenge for the firm run by investor Bill Gross.

A probe into pricing issues at Pimco has been under way for some months, the people familiar with the matter said. Gross has been interviewed by SEC investigators, the people said. Gross declined to comment for this article.