Giving evidence in court a week into the trial, the chief police investigator Ralph K. said the three Germans and a Turk were overheard by police discussing September 11, 2007 as a possible date for an attack.
They were also issued with emailed orders "to finish the job" by leaders of the Islamic Jihadic Union in Pakistan, an extremist group linked to Al-Qaeda, shortly before police captured three of them on September 4, 2007, he said.
And just one day before their arrest, when police caught them with enough equipment to make explosives 100 times more powerful than those used in the 2005 London bombings, the leaders told them to act within three weeks, he said.
Two of the suspects, Fritz Gelowicz and Daniel Schneider, are German converts to Islam, while Attila Selek - arrested later in Turkey - is a German of Turkish origin. The fourth is Adem Yilmaz, a Turkish national.
It is not just the timing of 9/11 that they are accused of wanting to emulate.
Prosecutors said last week at the start of their trial that they aspired to carry out attacks as big as those in 2001 in the United States which killed some 3,000 people.
Lawyers for the so-called Sauerland cell have called for the case to be thrown out, saying the state's evidence was based in part on testimony obtained in countries where witnesses may have been subjected to torture such as Uzbekistan and Kazakhstan.