British police have detained an 18-year-old suspect in connection with the bombing attack on the London underground the day before, Scotland Yard says.
The suspect was arrested in port city of Dover on Saturday, police said.
The search had been on for the suspect or suspects involved in leaving a bucket packed with nails and the explosive triacetone triperoxide (TATP) on a busy train during Friday's morning rush hour.
As of Saturday morning, officials were not sure whether more than one person had been involved in the bombing that - according to a new police report - left 30 people injured.
"Although we are pleased with the progress made, this investigation continues and the threat level remains at critical," Deputy Assistant Commissioner Neil Basu said.
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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.
