Garrett Bauer had been on the radar of securities regulators for years. The stock trader made big profits by betting on dozens of corporate take- overs but investigators could not figure out where he came by such reliable and accurate information.

Using a fairly crude computer program that analysed billions of trades collected over 25 years, investigators at the US Securities and Exchange Commission started running queries to find out who else traded the same stocks. A name came up: Kenneth Robinson, a stock and mortgage broker. The trading suggested the men knew each other but investigators could not prove illegal activity.

Finally in 2009, a breakthrough for the SEC led to Federal Bureau of Investigation agents knocking at Mr Robinson’s door. That year, he had bought shares in 3Com just before it was acquired by Hewlett-Packard. It was the only time both men had traded in advance of a deal where Wilson Sonsini Goodrich & Rosati, a law firm, acted as an adviser.