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.

The decision marks a milestone for plaintiffs lawyers Michael Freed of Freed Kanner London & Millen and Daniel Mogin of The Mogin Law Firm, who beat out Robbins Geller Rudman & Dowd and other firms to take the lead in the case soon after it was filed. The plaintiffs allege that the paper companies conspired to reduce supply and that they coordinated price hikes, forcing thousands of customers to overpay for boxes and other products in hundreds of thousands of transactions.