Police are continuing to hold a 10th person in connection with the murder of soldier Lee Rigby in Woolwich.
The 50-year-old man was arrested in Welling, south-east London on Monday, on suspicion of conspiracy to commit murder.
Police are also searching two addresses in south-east London and one address in south London.
Two suspects who were shot by armed police at the scene of the killing remain in hospital.
Michael Adebolajo, 28, and Michael Adebowale, 22 - both Britons of Nigerian descent who are understood to be converts to Islam - are in a stable condition.
Both are under arrest on suspicion of murdering Drummer Rigby.
The Metropolitan Police said the men would not be questioned until they had been discharged from hospital, and the time they had spent under arrest so far would not count towards the maximum amount of time they could legally be held without charge.
Of the eight other people arrested so far, five have been bailed and two released without charge.
Questions are still being raised about whether the security services could have done more to prevent the attack.
It has emerged that Mr Adebolajo and Mr Adebowale were known to the security services, and the UK Foreign Office confirmed it had given consular assistance to suspect Mr Adebolajo when he was arrested in Kenya in 2010.
Mr Adebolajo's childhood friend, Abu Nusaybah, also told BBC Newsnight that MI5 had once asked Mr Adebolajo to work for the agency, but that he had rejected the approach.
Parliament's Intelligence and Security Committee is to investigate the security service's actions in relation to the case.
The committee's chairman, Sir Malcolm Rifkind, told Radio 4's Today programme that he was "very confident" the committee would get to the bottom of the matter.
He had already had a conversation with Andrew Parker, the head of MI5, who he said "wishes to co-operate very fully with the investigations by the committee".
He said the committee now had the powers to "not simply ask questions of MI5... we can have access to all the relevant files, internal papers and any documents that are relevant to this matter".
Sir Malcolm said the committee would report to the prime minister and then to Parliament, and that report would also be public. Elements were likely to be redacted or left out, he said, "but only on grounds of national security".
"That will not prevent us reaching conclusions on whether MI5 have acted reasonably, satisfactorily or whether there was any defects in their approach."
He said MI5 was not "in the dock", and said there had been at least one terrorist plot a year since 7 July 2005 which had been prevented from killing British citizens, so "the record over the last few years has been hugely impressive".
The public have showed their support for Drummer Rigby and his family with thousands of flowers laid in his memory at the scene of his killing and the barracks in Woolwich where he was based.
And the Armed Forces Memorial Trustees announced that Drummer Rigby's name will be added to the Armed Forces Memorial at the National Memorial Arboretum.
A statement said: "Fusilier Lee Rigby was killed as a result of terrorist action outside Woolwich Barracks in London on 22 May 2013 and qualifies for inclusion on the Armed Forces Memorial.
"His name, along with other members of the UK armed forces who are killed on duty or through terrorist action in 2013, will be engraved in time for the Annual Service of Dedication in mid-2014."
In other developments, police are investigating after two monuments in central London - the Animals in War memorial in Park Lane and the RAF Bomber Command memorial in Green Park - were defaced with the word Islam in red spray paint.
And two men have been charged after a mosque was attacked with a petrol bomb in Grimsby, Lincolnshire.