Former financier Ivan Boesky, who illegally amassed a fortune on Wall Street, has returned to his plush California jail after a few days in New York where he was believed to have testified against another arbitrager.

Lt. Frederick Romaine told Reuters on Saturday that Boesky, convicted last year on insider-trading charges, left on Friday for the country-club prison 150 miles north of Los Angeles."Ivan Boesky was transferred to Lompoc Federal Prison," Romaine said from the Metropolitan Correctional Center in New York City. "They took him by air."

Legal sources said Boesky, the government's key witness in its ongoing insider trading probe, was in New York to testify against John Mulheren, former head of the arbitrage firm Jaime Securities.

The U.S. attorney is investigating charges by Boesky that Mulheren engaged in a "stock parking scheme," which entails hiding a stock's owner during buying or selling.

Mulheren has been charged with threatening the lives of Boesky and a former employee who is also cooperating with the government.

The government has asked for an extension of its Tuesday deadline on seeking an indictment against Mulheren, according to sources familiar with the investigation. Previous extensions have been granted, but there has been speculation in the legal community that the government's latest request would be denied.