Mutual fund family Oakmark has had enough of Dell (UNKNOWN:DELL.DL). In a statement released today, portfolio manager William Nygren outlined the reasons his company is selling its stake in Dell.

In a word: Blackstone (NYSE:BX). After the private equity firm pulled out of its $25 billion bid on April 18, Oakmark began to see a fundamental flaw with its own investing thesis:

"Our numbers suggested Dell should be worth a premium to the $13.65 offer from management, but then a potential acquirer with access to non-public information decided to end its quest to acquire Dell at a higher price. Since they had information we didn't, we believed it was prudent to assume they might be right," Nygren said in his statement.

Nygren mentioned that, in addition to information Blackstone would have received as a bidder for Dell, the computer company's own former head of mergers and acquisitions now works for Blackstone. Oakmark had high expectations for Dell's future acquisitions to expand its non-PC business -- expectations that seemed less warranted after Blackstone's bail.

According to The Wall Street Journal, Oakmark funds collectively owned approximately 24.5 million shares of the computer company, equivalent to 1.4% of total shares outstanding .

Fool contributor Justin Loiseau has no position in any stocks mentioned. You can follow him on Twitter, @TMFJLo, and on Motley Fool CAPS, @TMFJLo.

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