In a sweeping rejection of Apple Inc's strategy for selling electronic books on the internet, a US federal judge ruled that the company conspired with five major publishers to raise the retail prices of ebooks.
US District Judge Denise Cote in Manhattan found "compelling evidence" that Apple violated federal antitrust law.
She said it played a "central role" in a conspiracy with the publishers to eliminate retail price competition and raise ebook prices.
The decision could expose Apple to substantial damages. It is a victory for the US Department of Justice and the 33 US states and territories that brought the civil antitrust case.
Apple was accused of pursuing the conspiracy to undercut online retailer Amazon.com Inc's ebook dominance, causing some prices to rise to $12.99 or $14.99 from the $9.99 that Amazon charged.
Amazon once held a 90% market share.
"Apple chose to join forces with the publisher defendants to raise ebook prices and equipped them with the means to do so," Judge Cote said in a 159-page decision.
"Without Apple's orchestration of this conspiracy, it would not have succeeded as it did."
The decision was not a total surprise, given that Ms Cote indicated before the 2.5 week non-jury trial began on 3 June that Apple's defences might fail.
Ms Cote ordered a trial to set damages.
"This result is a victory for millions of consumers who choose to read books electronically," Bill Baer, head of the Justice Department's antitrust division, said in a statement.
"This decision by the court is a critical step in undoing the harm caused by Apple's illegal actions."
In a statement, Apple maintained that the plaintiffs' allegations are false and said it will appeal Ms Cote's decision.
"Apple did not conspire to fix ebook pricing," Apple spokesman Tom Neumayr said.
"When we introduced the iBookstore in 2010, we gave customers more choice, injecting much needed innovation and competition into the market, breaking Amazon's monopolistic grip on the publishing industry. We've done nothing wrong."
Last year, Apple settled a separate antitrust case over ebook pricing with the European Commission, without admitting wrongdoing.
The alleged collusion began in late 2009 and continued into early 2010, in connection with the Silicon Valley giant's launch of its popular iPad tablet.
Only Apple went to trial, while the publishers agreed to pay more than $166m combined to benefit consumers.
The publishers included Lagardere SCA's Hachette Book Group Inc, News Corp's HarperCollins Publishers LLC, Pearson Plc's Penguin Group (USA) Inc, CBS Corp's Simon & Schuster Inc and Verlagsgruppe Georg von Holtzbrinck GmbH's Macmillan.
Mr Baer said Judge Cote's decision, together with the publishers' settlements, have helped consumers by reducing prices of ebooks.
Steve Berman, a partner at Hagens Berman Sobol Shapiro pursuing consumer class-action litigation against Apple, called Cote's decision "a very big deal."
"It exposes Apple to hundreds of millions of dollars in damages, which is what we'll ask for," Mr Berman said.
Amazon's strategy involved buying ebooks at wholesale and then selling them at below cost, in an effort to promote its Kindle reading device.
Apple, in contrast, entered into so-called "agency agreements" in which publishers were able to set higher prices and pay commissions to the Cupertino, California-based company.
The federal government said this arrangement pushed Amazon into a similar model, and resulted in prices of ebooks from the five publishers increasing by 18%.
Evidence in the case included emails from Apple's late co-founder Steve Jobs to News Corp executive James Murdoch that the government said reflected Mr Jobs' desire to boost prices and "create a real mainstream ebooks market at $12.99 and $14.99".
Such evidence hurt Apple's case, Judge Cote said. "Apple's efforts to explain away Jobs's remarks have been futile," she said.
Apple had argued that it never conspired with the publishers to raise ebook prices, or even understood that publishers might have been talking among themselves about higher prices in advance of the iPad launch.
"There is no such thing as a conspiracy by telepathy," Apple's lawyer Orin Snyder said in closing arguments on 20 June.
Judge Cote also rejected Apple's argument that it would be unfair to single out the company when Amazon and Google Inc, among others, entered similar agency agreements with publishers.
The decision allows the plaintiffs to seek injunctive relief to prevent further pricing conspiracies.
At trial, the Justice Department said it wanted to block Apple from using the agency business model for two years.
department also said it wants to stop Apple over a five-year period from entering contracts that insure it will offer the lowest retail prices.