The directors of the publishing group Hollinger International have greatly increased the damages they are seeking from former chairman Conrad Black.
Lord Black was ousted as chief executive in November
They are questioning the way he sold some of the group's newspaper titles and are trying to claim $1.25bn (£700m) in compensation.
Hollinger, which owns the London-based Daily Telegraph, ousted Lord Black as chief executive in November.
The initial complaint it filed in January sought $200m in damages.
The BBC's North America business correspondent Steve Evans says Hollinger will claim in a Chicago court that Black sold newspapers he owned to other parts of his empire, essentially to himself, and then took exorbitant multi-million dollar management and other fees for the deals.
The amended lawsuit has increased the damage claims because of the alleged racketeering, the company said.
The firm, which also owns the Jerusalem Post and the Chicago Sun-Times, is selling key titles such as the Daily Telegraph.
It recently announced that it was delaying the publication of its latest results until the conclusion of the ongoing probe.
So far it has spent more than $8m (£4.5m) on an inquiry into the media tycoon's affairs.
Lord Black, who remains the company's controlling
shareholder, has denied any wrongdoing and has counter sued.