It makes a chilling picture. The mastermind behind the September 11 attacks has told interrogators that he and his terrorist nephew leafed through almanacs of American skyscrapers when planning the operation.

Sears Tower in Chicago and Library Tower in Los Angeles — which was “blown up” in the film Independence Day — were both potential targets, according to transcripts of interrogations of Khalid Sheikh Mohammed, the Al-Qaeda operations chief.

“We were looking for symbols of economic might,” he told his captors.

He recounted sitting looking at the books with Ramzi Yusuf, his nephew by marriage, who was the man behind the first World Trade Center bombing in 1993. In that attack Yusuf succeeded only in ripping a crater into the foundations with a van bomb. “We knew from that experience that explosives could be problematic,” said Mohammed, “so we started thinking about using planes.”

When he was captured in March last year in the house of a microbiologist in Rawalpindi, the paunchy 37-year old was unshaven and wearing a baggy vest. He looked more like a down-and-out than one of the most dangerous men in the world.

The interrogation reports make clear, however, that he was not only the chief planner for September 11 but also introduced Osama Bin Laden to Hambali, the Indonesian militant accused of the Bali bombing.

To date, Mohammed is the most senior Al-Qaeda member to have been caught. Until now there has been no word of where he is being held or what, if anything, he is saying.

Although the interrogation transcripts are prefaced with the warning that “the detainee has been known to withhold information or deliberately mislead”, it is clear that he is talking — and that the September 11 conspiracy was much more extensive than has previously been revealed.

The confessions reveal that planning for the atrocity started much earlier than anyone had realised and was intended to be even more devastating.

“The original plan was for a two-pronged attack with five targets on the East Coast of America and five on the West Coast,” he told interrogators. “We talked about hitting California as it was America’s richest state and Bin Laden had talked about economic targets.”

Bin Laden, who like Mohammed had studied engineering, vetoed simultaneous coast-to-coast attacks, arguing that “it would be too difficult to synchronise”.

Mohammed switched to two waves: hitting the East Coast first and following up with a second attack. “Osama had said the second wave should focus on the West Coast,” he said.

Zacarias Moussaoui, a French-Moroccan who had lived in London, was sent to the Pan Am international flight school in Minnesota to train for the West Coast attack, according to Mohammed. His instructor alerted the FBI, however, after the Moroccan showed no interest in landing planes — only in steering them. He was arrested in August 2001.

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