The game, initially given an "Adults Only" rating by the Entertainment Software Rating Board, went on sale in the U.S. on Wednesday with a "Mature" rating, after being modified. Most stores refuse to carry "Adults Only" games; Mature means a game is intended for player 17 or older.
Game publisher Take-Two Interactive Software Inc. and the studio that designed the game, Rockstar Games, have long been at the center of the debate over video game violence and children.
Two years ago, a hacker uncovered a hidden sex scene in their game "Grand Theft Auto: San Andreas."
In "Manhunt 2," the player takes the role of a man who escapes from an insane asylum and goes on a killing spree.
Take-Two edited parts of the game, including blurring some of the most gruesome killing scenes, to get the less restrictive rating.
Hackers defeated that blurring on the version of the game for Sony Corp.'s PlayStation Portable. The game is also available for the PlayStation 2 and Nintendo Wii systems, and those versions do not appear to have been hacked.
The hack does not roll back all the changes that enabled the game to qualify for the "Mature" rating, and it requires some technical expertise and a PSP unit that is itself hacked to accept modified software.
But Common Sense Media, a San Francisco nonprofit that advises parents about entertainment that may be inappropriate for children, Thursday asked the Federal Trade Commission to look into the ratings process, now funded and governed by an industry association. The process lacks basic transparency, Common Sense Media CEO James Steyer said in a statement.
"We believe that families and all consumers should have an assurance from game publishers and the game ratings board that the content being advertised is the same as the content being sold," Steyer said.
In the Grand Theft Auto incident, the ratings board changed the game's rating from "Mature" to "Adults Only" and retailers pulled it off shelves.
Since then, the board has required that publishers submit even hidden content for review, and Take-Two spokesman Ed Nebb said the publisher had followed that requirement for "Manhunt 2."
It is unclear whether the private, nonprofit ratings board considered the hidden material in assigning the "M" rating to "Manhunt 2."
Board spokesman Eliot Mizrachi said only that it is aware of the hacking issue and is looking into it.
Both the revised and original versions of "Manhunt 2" were banned by the American ratings board's British counterpart.
"I stand behind the game and the ESRB ratings process," Take-Two Chairman Strauss Zelnick said in a statement. "It is unfortunately the case that no one in the entertainment software industry is immune from hacking. We hope that consumers will not engage in hacking or download illegally modified copies of our games."