Japanese regulators reached breaking point in March after months of stonewalling by the country's largest broker, which they suspected of leaking confidential information to clients ahead of share sales.

A trail of evidence in a near 2-year probe into pre-sale tip-offs, the most extensive in Japan for years, had taken investigators deep inside Nomura Securities, part of the 87-year-old group (8604.T) at the heart of Japan's capital markets, but the firm's top executives kept stalling, say people involved in the investigation.

In late March, bankers waiting outside the office of Shozaburo Jimi, then Financial Services Minister overseeing the regulatory agency, were startled to hear him bellow to aides: "I haven't had one call from Nomura yet. What the hell's going on?"

From interviews with more than a dozen bankers, regulators and lawyers with direct knowledge of the investigation emerges a picture of a reluctant watchdog roused into action in 2010 by whistleblowers with statistical evidence that the Tokyo market was rigged against share issuers and investors.