Police in Australia are still holding one Indian doctor, Mohamed Haneef, suspected of links with the plot.
''It's not a question of whether people are in the clear or not, it's a question of whether or not you have come to a view that they would continue to be available for questioning that might arise during the course of the inquiry,'' he told Nine Network television.
''The distinguishing factor in relation to Dr Haneef from the others was that he was intent on leaving Australia ... he had a one-way ticket.'' Haneef was detained on Monday while trying to leave Australia.
His family in India has said he is innocent and brother Mohammed Shuaib said Haneef had only planned to visit his wife Firdaus Arshiya, who recently gave birth in Bangalore.
Ruddock also played down unsourced media reports suggesting that there was a plan to detonate bombs in front of a London nightclub by mobile phone from Australia.
''It probably misstates what is in the public arena,'' he said.
''That is that some of the people who had been in the United Kingdom who had now come to Australia as temporary residents left behind telephones and SIM cards which other people were using.
''I'm not sure there is a direct connection but look, I wouldn't want to foreclose any avenue of inquiry that the police are taking.'' The Australian Federal Police dismissed the media reports.
''The AFP advises that media reporting that a device in London was to be detonated from Australia is not accurate,'' a spokesman said.
''There is no evidence to substantiate these media claims,'' he said.
Invoking anti-terrorism powers, an Australian judge on Thursday granted Australian police and a British counter-terrorism officer a further four days to question Haneef.
Police are combing more than 30,000 files on Haneef's laptop computer and a mobile phone SIM card device Haneef left with one of the British bomb suspects.