A seventh person has been arrested in connection with the Parsons Green bombing, Scotland Yard said.
The 20-year-old man was seized by officers in Cardiff at around 6am on Monday.
He has been taken to a police station in south London and is in custody under section 41 of the Terrorism Act.
Officers said they are carrying out searches at addresses in Cardiff and Surrey.
Ahmed Hassan, 18, has been charged over the attack on a packed District Line Tube train in London.
The Iraqi asylum seeker appeared at Westminster Magistrates' Court on charges of attempted murder and using the chemical compound TATP to cause an explosion that was likely to endanger life.
Hassan allegedly bought parts to make the improvised explosive device online and left the train at Putney Bridge before the device partially detonated at the next station.
Witnesses described how a fireball erupted inside the carriage, leaving 30 people injured, including one woman with serious burns.
Hassan, of Cavendish Road, Sunbury, Surrey, was arrested at the port of Dover in Kent at 7.50am on Saturday September 16 by counter-terrorism officers.
The teenager, whose parents are understood to have been killed in Iraq, was in the foster care of Penelope and Ronald Jones, aged 71 and 88 respectively, who previously received MBEs for services to children and families.
Five others have been arrested over the bombing on September 15.
A 25-year-old and a 30-year-old remain in police custody.
Police have released three others - a 17-year-old boy and two men aged 21 and 48 - without further action.