A sixth man was arrested in Britain early on Thursday as part of a probe into the bombing attack on a London Underground train last week, police said.

The man, aged 17, was detained under anti-terrorism powers in Thornton Heath, south of London, where a search is under way, a statement said.

Dean Haydon, an anti-terrorism officer, said the investigation was progressing rapidly.

Some 30 people were injured in the September 15 bomb attack at Parsons Green station in west London, which was claimed by the militant Islamic State group.

The bomb had been hidden in a large white bucket. It apparently failed to detonate fully during morning rush hour but resulted in what witnesses described as a “fireball”.

Some commuters suffered burns, while others were injured in the stampede to the exit that ensued.

The bombing was the fifth terror attack in Britain in six months, which combined have claimed 35 lives.

The terror alert in Britain was raised to its maximum “critical” level on Friday, meaning another attack was believed to be imminent. It was lowered back to “severe” on Sunday.

As part of the police investigation, an 18-year-old man, reportedly an Iraqi refugee, was arrested in the ferry departure terminal of the port of Dover on Saturday. A 21-year-old man was detained later the same day in west London.

The probe has focused on a foster home in Sunbury-on-Thames, a suburb of London where the 18-year-old is believed to have lived.

A third man, aged 25, was detained by anti-terror officers in Newport, Wales on Tuesday and two more arrests were made in the same city on Wednesday.