The Metropolitan Police says a 21-year-old man was detained under Britain's Terrorism Act in the West London suburb of Hounslow. Intelligence officials say there is scant evidence Daesh was behind the attacks.
Detectives from the Metropolitan's Counter Terrorism Command investigating Friday's bomb attack on a London underground train arrested a second man, police said on Sunday.
Britain was on its highest level of alert with soldiers helping provide security after the attack at Parsons Green that injured 30 people on Friday. But it lowered the national security threat level to severe-the second highest level, means an attack is highly likely- from critical on Sunday.
The 21-year-old man was detained under Britain's Terrorism Act in the west London suburb of Hounslow just before midnight on Saturday, the Metropolitan Police said in a statement.
A second man has been arrested by detectives investigating the terrorist attack at Parsons Green on Friday https://t.co/3fyNA1fh1L— Metropolitan Police (@metpoliceuk) September 17, 2017
"We are still pursuing numerous lines of enquiry and at a great pace," counter-terrorism coordinator Neil Basu of the London police said late Saturday.
TRT World speaks to Olly Barratt in London.
Not a lone-wolf attack
Police arrested an 18-year-old teenager in the port of Dover earlier on Saturday and then raided a property in Sunbury, a town near London and about six kilometres (four miles) from Hounslow.
The teenager was detained in the departure lounge of the port of Dover in what police said was a "very significant" step.
The home-made bomb shot flames through a train carriage packed with commuters at west London's Parsons Green Tube station but apparently failed to detonate fully.
Daesh claimed responsibility for the attack.
It also claimed responsibility for other attacks in Britain this year, including two in London and one at a concert by American singer Ariana Grande in Manchester in May.
Intelligence officials say there is scant evidence the militant group was behind the attacks.
Interior minister Amber Rudd said on Sunday the second arrest indicated it was not a "lone-wolf" attack, but there was no evidence Daesh was involved.
"It is inevitable that so-called Islamic State, or Daesh, will reach in and try to claim responsibility. We have no evidence to suggest that yet," she told the BBC's Andrew Marr Show.
"But as this unfolds, and as the police do their investigations, we will make sure that we find out exactly how he was radicalised, if we can."
Security threat level
Britain lowered its national security threat level to severe from critical on Sunday after an investigation into a bomb attack on the London Underground progressed, Rudd said.
"Following the attack on Parsons Green last Friday, the police have made good progress with what is an ongoing operation," she said.
"The Joint Terrorism Analysis Centre, which reviews the threat level the UK is under, has decided to lower that level from critical to severe."
Prime Minister Theresa May put Britain on its highest security level of "critical" late on Friday, meaning another attack might be imminent.
Soldiers and armed police were deployed to strategic locations such as nuclear power plants.
The last time Britain was put on "critical" alert was after a suicide bomber killed 22 people at the Ariana Grande concert.
On that occasion, the threat level remained at the highest setting for four days while police raced to establish if the bomber had worked alone or with the help of others.
Prior to that it had not been triggered since 2007.