A 55-year-old man and 53-year-old woman were released on Monday.
London police say they can not confirm the names of people caught up in the attack.
Butt, 27, was apparently born in Pakistan while Redouane, 30, claimed he was Moroccan and Libyan.
"This has been a truly ghastly few weeks", said London police chief Dick, who added the spell of recent attacks was unprecedented in her working experience, which began in 1983.
Police searched more addresses in east London on Monday and said a "number of people" had been detained following the raids in Newham and Barking.
"Indeed I would. Because there have been calls made by a lot of very responsible people on this, who are very anxious that she was at the Home Office for all this time, presided over these cuts in police numbers and now is saying that we have a problem".
The Islamic State group claimed responsibility for the attack through its Amaq news agency.
May said Sunday that three terrorist attacks in Britain in the last three months are "bound together by the evil ideology of Islamist extremism". When questioned by police at the airport about the reasons for his journey, he replied: "I'm going to be a terrorist".
The nearby London Bridge rail station also reopened after police cordons around it "were lifted earlier than expected", according to transportation officials, but police said the public should expect to see extra officers on duty during Monday's commute. Both of the men resided in Barking, an East London suburb.
Valeria Khadija Collina, who lives near Bologna in Italy, said she had told police her son was missing after Saturday's attack.
"There are 500 current terrorism investigations, involving 3,000 current subjects of interest".
A statement from the shelter where Archibald had worked said "Chrissy was a bright light to many, and her generosity, kind spirit and huge heart for her work in responding to issues of addictions and homelessness at the centre inspired us all".
London Mayor Sadiq Khan, meanwhile, called the incident a "cowardly attack on innocent Londoners and visitors to our city enjoying their Saturday night".