Yahyah Farroukh, was arrested in Hounslow, London, late on Saturday evening after the explosion on a tube train that took place early on Friday morning, injuring 30.
No-one was seriously hurt when the improvised explosive device contained in a supermarket bag detonated at the height of rush hour at Parsons Green, in South West London.
Farroukh, who is from Syria, was pictured on a ferry in the aftermath of his arrested, which was later revealed to have been taken on the Rothesay-Wemyss Bay ferry on the Clyde.
He is believed to have been living with a couple who were honoured for their services to children, and was said to have visited relatives in Scotland.
They described the 21-year-old in local press reports as a ‘good guy’ who liked smoking, girls and nightclubs as they expressed shock at his arrest.
A Metropolitan Police statement confirmed that Farroukh had been released with no further action.
Commander Dean Haydon, head of the Met Counter Terrorism Command, said: “This continues to be a fast-moving investigation. A significant amount of activity has taken place since the attack on Friday.
“We have five males in custody and searches are continuing at four addresses. Detectives are carrying out extensive inquiries to determine the full facts behind the attack.