Three people, including the widow of 7 July bomber Mohammad Sidique Khan, have been released without charge after being questioned over the 2005 attacks.
Mohammad Sidique Khan bombed a Tube train on 7 July 2005
Hasina Patel, 29, brother Arshad Patel, 30, and Imran Motala, 22, were held on 9 May in West Yorkshire and Birmingham.
Police have been granted a warrant to continue to detain Khalid Khaliq, 34, of Beeston, Leeds, until next Monday.
The four had been held in London on suspicion of commissioning, preparing or instigating acts of terrorism.
Last week's arrests followed an intelligence-led operation involving Scotland Yard's counter-terrorism command.
Police searched seven addresses in Dewsbury, Batley and Beeston, all in West Yorkshire, and Birmingham.
A spokesman for Scotland Yard said: "In all operations some people may be released early without charge while others may remain in custody for further investigation.
"This is not unusual and is to be expected in large and complex criminal investigations."
'Relieved and shocked'
Mrs Patel's solicitor, Imran Khan, said he was "relieved" at her release, but remained "shocked" at the police's handling of her arrest.
He told the BBC the police had been for some time in possession of evidence that "unequivocally" proved she had known nothing about what her husband was planning two years ago.
He added: "To arrest her in these circumstances - a woman who lost her husband, who has been accused of the most atrocious events that have taken place in this country, has now spent seven days in isolation in Paddington Green - I wonder what she must be feeling.
"She's quite clearly innocent of anything, because she's been released by police having trawled through her life and possessions and caused her a tremendous amount of grief.
"Those in her community are incredibly angry at the way the police have approached this."
Masood Ahmed, Labour councillor for Dewsbury South, in West Yorkshire, said the releases showed the three "had nothing to do with the bombings".
A London bus and three Tube trains were targeted on 7 July
He added: "There will be some reaction, I am sure, but I think the incident was much better dealt with by the police than the original situation."
Fifty-two people died when Sidique Khan and three other suicide bombers attacked the London transport system on 7 July 2005.
Last month the first people to be charged in connection with the bombings appeared at the Old Bailey in central London.
Mohammed Shakil, 30, Sadeer Saleem, 26, and Waheed Ali, 23, of Beeston, Leeds, are accused of conspiring with the four bombers to cause explosions.
The trio, who came before the court via a video link from prison, were remanded in custody until 8 June.