British police are holding two suspects in custody over the London subway blast as commuters head to work in the first morning rush hour since the attack.
Authorities have reduced the terror threat level from "critical" to "severe," indicating another attack is highly likely but no longer believed to be imminent.
Closed-circuit television images broadcast Sunday appear to show a person walking with a Lidl shopping bag in the suburb of Sunbury before Friday morning's attack at Parsons Green station that injured 30 people in southwest London.
Images posted on social media following the attack appeared to show wires protruding from a flaming bucket contained in a Lidl bag on the floor of the train carriage.
The bomb only partially exploded. Officials say the injuries would have been far worse if it had fully detonated.
Police haven't charged the two men, aged 18 and 21, arrested in the case. They are being questioned in a south London police station.
Both are believed to have spent time in the care of Penelope and Ronald Jones, aged 71 and 88 respectively, who previously received MBEs for services to children and families.
The 21-year-old, reported to be from Syria, has since left their care and was arrested in Hounslow on Saturday night, while his home in Stanwell, Surrey was searched by police on Sunday.
A local politician said he understood an 18-year-old Iraqi orphan was living with the couple, having moved to Britain aged 15 after his parents died.
The younger man was arrested in the ferry departure area in the port of Dover on the English Channel. The 21-year-old was arrested in west London. Both are being held under the Terrorism Act.
Police have searched several properties, including a home in Sunbury where the elderly couple served as foster parents for many children over several decades.
Most of the injured in Friday's explosion aboard a District Line train suffered flash burns while some were injured in the panicked rush to leave the Parsons Green station where the blast occurred.
The Islamic State group has claimed responsibility, but British officials say there is no proof yet that it was involved.