Home Secretary Amber Rudd today said that it seemed the Parsons Green bomber was not a lone wolf, but added: "It's too early to reach any final conclusions on that".
Soldiers who had been drafted in to guard public sites in order to free up police resources will "return to their original positions" over the next few days, Ms Rudd said.
Just one person is being treated at Chelsea and Westminster Hospital, which has a specialist burns unit.
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The country's top counter-terrorism officer, Assistant Commissioner Mark Rowley, said police were gaining a "greater understanding of the preparation of the device".
The second arrest comes after an 18-year-old - the youngest to be arrested over a terror attack in the United Kingdom - was quizzed by cops after he was seized in Dover.
British police are searching a home in the London suburb of Stanwell linked to the arrest of the second suspect in the subway bombing.
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The couple, Ronald Jones, 88, and his wife Penelope, 71, who were honoured for services to children and families, are said to be staying with friends following the police raids.
The entire neighborhood around their house was evacuated as a precaution when the search began Saturday.
Rudd's comments have echoed those of Theresa May, who on Friday said it was not helpful for anyone to speculate on active investigations.
It also follows the arrest of an 18-year-old at the Port of Dover and a 21-year-old in Hounslow, West London. He is being questioned at a south London police station but has not been charged or identified.
Her announcement came after two people were arrested in connection with the blast.
The terror threat level has been reduced from "critical" to "severe" after a teenager arrested at the Port of Dover was among two taken into custody in conjunction with the Parsons Green Tube terror attack.