After striking a deal with the U.S. Attorney's Office, former Hollinger second-in-command David Radler must now co-operate with a Justice Department probe into his former boss and friend Conrad Black.

Radler, 63, pleaded guilty in a Chicago court Tuesday to one count of mail fraud.

He agreed to a 29-month jail term and was fined $250,000 US for his role in an alleged scheme to swindle more than $32 million US from Hollinger International Inc., Black's former newspaper operating company.

Radler's lighter-than-expected sentence reflects the valuable information Radler can provide investigators, Jeffrey Gandz, of the Richard Ivey School of Business in London, Ont., told The Associated Press.

If Black is found guilty, it is clear his sentence "will be considerably longer than 29 months," Gandz said.

No charges have yet been laid against Black, but prosecutors are using Radler's co-operation to work up an iron-clad case against the former Hollinger boss.

Black, 61, who founded Hollinger and built it into what once was the world's number three publisher of English-language newspapers, is fighting a lawsuit from the US Securities and Exchange Commission and a $425 million US civil lawsuit by Hollinger that claims he looted the company through excessive, undisclosed payments to himself and associates.

Jacob Frenkel, a partner with Washington, D.C.-area law firm Schulman, Rogers and a former U.S. federal prosecutor, told AP "if (Black is) charged, if there's a trial, and if there is a conviction, we would be looking at a sentence along the high end of the corporate executives who already have been sentenced in the United States -- into two decades."

Radler has already been thoroughly debriefed and has likely provided investigators with significant leads to pursue, Frenkel said.

"They probably have squeezed out of him 80 per cent of the information that he knows independently," he added.

U.S. Attorney Eric Sussman, who is prosecuting the case, declined to comment on Radler's co-operation, but said that Radler will be expected to go to Chicago from his Vancouver home for "any trial, or pre-trial, preparations."

However it could be months before prosecutors assemble their argument as the Hollinger case is full of business complexities.

"Given the difficulty that we saw in the Martha Stewart case of the thing becoming too complex, such that a jury gets confused by it or turns off by it, they will want to plan the approach very closely," Gandz told AP.

"The interesting question is, if Conrad Black is charged, will he be tried by U.S. authorities or Canadian authorities?" Frenkel added, noting that nothing had been heard from Canadian authorities.

"This is a tremendous opportunity for the Canadian authorities to take an aggressive posture," he told AP.

AP reported that a spokesman for the Ontario Securities Commission could not be reached for comment Wednesday and that a spokesman for Black declined to discuss the case.

According to Gandz, prosecutors will almost certainly rely on Radler's word for much of the evidence, pitting Black's former lieutenant against him in verbal battles.

"Many criminal trials are based on he said, she said," Gandz said. "And it's all a matter of who a jury believes."

Radler, a native of Montreal who now lives in Vancouver, may be permitted to serve his 29-month prison sentence in Canada.

The U.S. Government agreed not to take a position if Radler filed an application for the international prisoner transfer program.