Five years into litigation over an alleged price-fixing scheme by the biggest players in the paper and packaging industry, plaintiffs lawyers gained ground on Thursday when a Chicago federal judge allowed the case to move forward as a class action.

In a sealed ruling, U.S. District Judge Harry Leinenweber certified a class of purchasers of containerboard and corrugated products, such as boxes, who bought directly from Georgia-Pacific LLC, International Paper Co., Weyerhaeuser Co. and other companies at allegedly inflated prices between February 2004 and November 2010.