Khalid Sheikh Mohammed's Vacuum Challenge

The mastermind of the 9/11 terrorist attacks may have been given an unusual mission while confined to a CIA secret prison - to redesign the humble vacuum cleaner.

A former secret service official has revealed to the AP news agency that Khalid Sheikh Mohammed asked his American jailers in Romania whether he could use his expertise in mechanical engineering to rework the household appliance.

He allegedly endured brutal interrogation methods, including being forced to stay awake for 180 hours and 183 instances of waterboarding in Poland.

In that time, he confessed to a career of atrocities.

But as his intelligence value diminished, the CIA in Romania tried to find ways to occupy him.

"We didn't want them (CIA prisoners) to go nuts," the former official said.

He was given assignments about his knowledge of al Qaeda, or "homework," as CIA officers called it.

He received Snickers bars as a reward and was allowed to read the entire Harry Potter series.

But it remains unclear whether Mohammed was really interested in designing a better vacuum or had ulterior motives.

He may have been inspired by Graham Greene's spy thriller Our Man In Havana, in which a vacuum salesman in Cuba dupes MI6 into believing his designs are military installations.

And it is a mystery how far Mohammed got with his designs.

His military lawyer, Jason Wright, said he was prohibited from discussing his client's interest in vacuums.

"It sounds ridiculous, but answering this question, or confirming or denying the very existence of a vacuum cleaner design, a Swiffer design, or even a design for a better hand towel would apparently expose the US government and its citizens to exceptionally grave danger," he said.

The CIA told the AP that the plans, "should they exist", would be considered classified operational files of the CIA and therefore exempt from ever being made public.

However, the CIA did apparently succeed in keeping Mohammed sane. He appears to be in good health, according to current military records.

The secret CIA prison in Romania was closed in 2006 and he was transferred to Guantanamo Bay - most likely without his blueprints.