A second man has been arrested by detectives investigating the terrorist attack at Parsons Green station, the Metropolitan Police has said.
He was taken to a south London police station and remains in custody.
An 18-year-old man was arrested in the departure area of the port of Dover in connection with the attack on Saturday morning.
He was arrested by Kent Police under the Terrorism Act and remains in custody at a London police station.
Deputy Assistant Commissioner Neil Basu described the arrest as very significant.
A search was also carried out at a house in Sunbury-on-Thames, Surrey, at the home of foster care couple Penelope and Ronald Jones, who previously received MBEs for services to children and families.
The scene of a terrorist attack at Parsons Green Tube station and (inset) the homemade bomb used The couple are said to be staying with friends for at least the next five days following the police raids, during which surrounding houses were evacuated by counter-terror officers, with residents told they had one minute to flee their homes.
Friends said the couple – foster parents for almost 40 years who had taken in up to 300 children, including eight refugees – had an 18-year-old and a 22-year-old staying with them recently.
The latest arrest comes after Mr Basu said police were keeping an “open mind” over whether more than one suspect was involved in the bombing.
He said the UK’s terror threat level would remain at “Critical,” the highest level, while officers investigated whether others were involved.
He added: “If there are other people responsible it’s our job to find them and that is part of the reason that we are remaining at critical threat.”
Thirty people were injured when the improvised device exploded during rush hour at Parsons Green station.
Police have since identified 121 witnesses, of whom 100 have been spoken to, Mr Basu said.
A key strand of the investigation has focused on CCTV as officers comb through footage to establish who planted the device, and when and where it was placed on the train.