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British police are treating an incident on a London Underground train on Friday as an act of terrorism, saying "a number of people" had been injured. "Terrorist incident declared at Parsons Green Underground Station," police said in a statement.
 / AFP PHOTO / Daniel LEAL-OLIVAS        (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)
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Police forensics officers works alongside an underground tube train at a platform at Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage at the station. British police are treating an incident on a London Underground train on Friday as an act of terrorism, saying "a number of people" had been injured. "Terrorist incident declared at Parsons Green Underground Station," police said in a statement. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)
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Members of the emergency services work alongside an underground tube train at a platform at Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage at the station. Police and ambulance services said they were responding to an "incident" at Parsons Green underground station in west London on Friday, following media reports of an explosion. "We are aware of an incident at Parsons Green tube station. Officers are in attendance," London's Metropolitan Police said on Twitter. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)
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(CNN) —  

A homemade bomb that partially exploded on a London Underground train last week was “packed with shrapnel” and could have wreaked devastation, London’s police chief said Friday.

Metropolitan Police Commissioner Cressida Dick said the bomb could have caused much more damage if it had fully detonated.

“That was a very, very dangerous bomb. It partially detonated, it had a large quantity of explosive, and it was packed with shrapnel. So, it could have been so much worse,” Dick told LBC, a London-based talk radio station.

Thirty people were injured when the bomb went off a week ago as a District Line train was halted above ground at Parsons Green station in a leafy area of southwest London.

Six men have been arrested in connection with the attack. Two were later released.

A 21-year-old man who was arrested Saturday and a 48-year-old man detained Wednesday were released Thursday “with no further action,” according to a police statement. Four others remain in custody under the Terrorism Act.

The Met Police said searches were continuing at four addresses and were expected to continue for some days.

“This continues to be a fast-moving investigation,” said Commander Dean Haydon, head of the Met Counter Terrorism Command. “Detectives are carrying out extensive inquiries to determine the full facts behind the attack.”

Speaking to LBC, Dick said the police had foiled six terror plots “in the past several months,” adding that the methods used by terrorists were always evolving.

“Different tactics will be used by different people at different times depending on what they want to achieve – we have to be prepared for everything,” she said.

The UK terror threat level was briefly raised to “critical” in the wake of last Friday’s attack but has subsequently been lowered to “severe,” meaning an attack is highly likely.