A Goldman Sachs Group Inc. director tipped off a hedge-fund billionaire about a $5 billion investment in Goldman by Warren Buffett's Berkshire Hathaway Inc. before a public announcement of the deal at the height of the 2008 financial crisis, a person close to the situation says.

The revelation marks a significant turn in the government's case against Raj Rajaratnam, the hedge-fund titan at the center of the largest insider-trading case in a generation. Mr. Buffett's investment in Goldman in September 2008 was a watershed moment...