The U.S. Securities and Exchange Commission is investigating IBM's accounting practices in the U.S., Britain and Ireland, the company revealed in a filing Tuesday. The inquiry, about which IBM learned in August, relates to how the company recognizes revenue. The tech giant has recorded declining revenue in each its last 14 quarters. 

Earlier in the day, the company announced a surprise $4 billion share repurchase program, expanding its buyback authorization to $6.4 billion in a gesture to shareholders. Yet at the close of trading Tuesday, IBM shares had fallen 4 percent, or $5.80, to $137.86, the lowest the stock has traded in five years. 

S&P Capital IQ kept its buy rating for IBM, citing the company's relatively attractive price-to-earnings ratio of 9 times earnings, well below the Standard and Poor's 500 average of 22.