A 19-year-old man was held on suspicion of preparing terrorist acts following a raid at an address in Barking, east London, on Sunday, Scotland Yard says.
Detectives from the Metropolitan Police's Counter Terrorism Command are searching the address and another in Barking where a 28-year-old man was arrested on Saturday.
A total of seven men are being held in custody after the June 3 attack that left eight dead and dozens wounded.
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Detectives have made 21 arrests as part of the probe.
Twelve people arrested in the initial stages were released without charge.
A 27-year-old arrested under the Terrorism Act on Tuesday was also released without charge on Saturday.
A man arrested on suspicion of drugs and firearms offences in connection with the investigation was bailed until a date in late June.
