London (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.
London police chief: Parsons Green train bomb 'packed with shrapnel'
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.