The Justice Department has joined a widening investigation of banks, private-equity firms and hedge funds that may have violated antibribery laws in their dealings with Libya's government-run investment fund, people familiar with the matter said.

The criminal investigation, which has intensified in recent months, is proceeding alongside a civil probe by the Securities and Exchange Commission that began in 2011 and initially homed in on Goldman Sachs Group Inc. The Justice Department's involvement hasn't been reported previously.

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