Rajat Gupta, the Goldman Sachs director who is being investigated by US authorities over his links to Galleon LLC founder Raj Rajaratnam, had a longstanding business relationship with the billionaire hedge fund manager.

Interviews, public records, lawsuits and regulatory filings show a 13-year history of co-investing and other business collaborations between Gupta, 61, the former worldwide head of consulting firm McKinsey & Co., and Rajaratnam, 52, the central figure in the Galleon insider trading probe.

Rajaratnam has a stake in a fund managed by New Silk Route NSR Partners LLC, founded by Gupta and three others in 2006 to invest in South Asian companies, according to a New Silk Route spokeswoman.

The fund owns stakes in at least 11 Indian companies, including cell phone tower operator Reliance Infratel Ltd. and the Cafe Coffee Day chain.