The senator wants the US Federal Trade Commission to find out who put the explicit material in the game.

The US Entertainment Software Rating Board is already investigating GTA maker Rockstar over the matter.

Rockstar has denied that it included the graphic content in San Andreas and has instead accused hackers of altering the code.

Test case

The controversy about the Hot Coffee mod broke earlier this week with the ESRB's announcement of its investigation.

The board wants to find out if Rockstar broke rules that demand it tell industry regulations if such material is included in games.

The ESRB wants to work out if the sexual content was in the game all along but hidden or if the mod adds it.

For its part Rockstar said that the Hot Coffee mod only works because hackers took apart, rebuilt and then recompiled the game's underlying computer code.

Hackers had gone to "significant trouble to alter scenes in the official version of the game," Rockstar said in a statement.

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Mr Wildenborg said all the scenes unlocked by the mod were in the original game but were simply inaccessible.

Rockstar said it was looking into ways to change San Andreas to stop the Hot Coffee mod working.

Concerns about minors

As well as seeking to find out who put the content in the game, Senator Clinton wants the FTC to find out if the game has been given the wrong rating by the ESRB.

Senator Clinton also wants to find out if game stores are doing enough to stop minors getting hold of the game.

In a letter to the FTC, Senator Clinton cited a study by the National Institute on Media and the Family that found 50% of boys between seven and 14 years old could buy mature rated games.

"There is no doubting the fact that the widespread availability of sexually explicit and graphically violent video games makes the challenge of parenting much harder," said the senator in her missive to the FTC.

The mature or "M" rated PC version of San Andreas is only supposed to be bought by those aged 17 and over.

If the game were to be get the rarely awarded Adults-Only (AO) rating, sales of the game could plummet. The PlayStation 2 version of San Andreas was the best-selling game of 2004 in the US.