British police have made a second arrest in relation to Friday’s bombing of a London train carriage.
A 21-year-old man was arrested in Hounslow before midnight Saturday (local time), the London Metropolitan Police Counter Terrorism Command said in a statement.
It follows the arrest of an 18-year-old man in Kent suspected in relation to the bombing that injured 29 people on a London train on Friday, which police described as "significant".
"We have made a significant arrest in our investigation this morning," Neil Basu, Senior National Co-ordinator for Counter Terrorism Policing, said in a statement.
"Although we are pleased with the progress made, this investigation continues and the threat level remains at critical."
Both men were arrested under section 41 of the Terrorism Act.
British police said earlier they were keeping an open mind on whether more than one person was responsible for the bombing.
"We are still pursuing numerous lines of enquiry, and at great pace," Mr Basu told reporters.
"Our priorities are ... to identify and locate any other potential suspects."
Police further reported speaking to 45 witnesses and that they were continuing to gather information.
The site of the attack, London's Parsons Green underground station, was re-opened in the early hours of Saturday.
Late on Friday, British Prime Minister Theresa May announced that the country's terrorism warning had been raised to critical, its highest level.
Friday's detonation was considered by police to have been partial.
There were fears the number of those hurt could have been much higher - with the real potential for life-threatening injuries - had the bomb, which was concealed within a supermarket carrier bag, fully exploded.