Thirty people were injured when a homemade bomb placed inside a bucket wrapped in a supermarket bag partly detonated on a train stopped at London’s Parsons Green station.
The attack sparked a manhunt for the perpetrators and prompted officials to briefly raise the national terrorism threat to the highest level.
The suspects still being held include an 18-year-old from Iraq detained at the English Channel port of Dover on Saturday.
Two men aged 25 and 30 arrested in Newport earlier this week remained in custody, as did a 17-year-old detained early Thursday in south London.
None of the suspects has been charged, and their names haven’t been released.