British detectives have arrested a 17-year-old youth in connection with a bomb attack on a London Underground train last week, bringing the total number of arrests to six, police said on Thursday.

The young man was arrested under the Terrorism Act in the early hours of Thursday in Thornton Heath, south London. The other arrests had taken place in Dover on the south coast of England, Hounslow in west London and Newport in Wales.

“This continues to be a fast-moving investigation. A significant amount of activity has taken place since the attack on Friday,” said Commander Dean Haydon, head of the Metropolitan Police’s Counter Terrorism Command.

“We now have six males in custody and searches are continuing at five addresses. Detectives are carrying out extensive inquiries to determine the full facts behind the attack,” he said.

A home-made bomb went off on Sept. 15 during the morning rush hour on a packed underground Tube train at Parsons Green station, sending flames through the carriage, although it appeared that the device did not fully explode.

The attack, which was the fifth major terrorism incident in Britain this year, left 30 people injured. Some commuters suffered burns, while others were injured in the stampede to the exit that ensued.