A sixth person was arrested in connection with the investigation into a bomb attack which took place on a London Underground train last week, British police said Thursday.

The 17-year-old suspect was detained in Thorton Heath, a suburb south of London, where a search was ongoing.

30 people were injured in the September 15th attack, when a homemade explosive device left on a London train partially detonated, shooting flames. The attack took place when the train was stopped at Parsons Green station and was claimed by the Islamic State of Syria and Iraq (ISIS).

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"This continues to be a fast-moving investigation. A significant amount of activity has taken place since the attack on Friday," Commander Dean Haydon, head of the Met Counter Terrorism Command, said in a statement.

"We anticipate that the searches will take some days to complete and may cause further disruption," he said.

The attack was the fifth terror attack in the United Kingdom in 6 months. It caused the national terrorism threat level to be raised to "critical," meaning an attack was judged to be imminent. The threat level was then lowered again after the first two arrests.