This is the fifth attack the police have classified as "terrorist" in the United Kingdom this year.
In what they are calling a "significant arrest", police in London said they have detained an 18-year-old man in connection with the explosion on a train Friday that left dozens injured. He said the investigation is ongoing and the terror threat level remains at "critical".
Prime Minister Theresa May said after the attack that the country's Joint Terrorism Analysis Center chose to raise the country's threat level to critical, meaning that a further attack may be imminent.
The organization said via its Amaq news agency that a "detachment" from the group had carried out the attack.
"The public should remain vigilant as our staff, officers and partners continue to work through this complex investigation", he said.
British police say armed officers are also searching a home in a London suburb and evacuating neighbors as precaution.
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.
Images from inside the train vehicle after the blast showed that the device was contained in a bucket with wires hanging out of it and that it was concealed in a plastic shopping bag.
Parsons Green station has re-opened after the attack.
With the person or persons responsible for yesterday's blast still at large, the immediate concern now is ensuring no further attacks take place.
She said that the public could expect to see more armed police patrolling the streets and the transport network in the coming days.
In the immediate aftermath of a bombing on a London train Friday, Trump tweeted that the United Kingdom government "must be proactive" in its search for suspects and said that police had known of the perpetrators.
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