British police are still holding two suspects in custody over the London subway blast as commuters head to work Monday 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. Police have asked the traveling public to be vigilant and to expect to see an increase in armed police.

Meanwhile, closed-circuit television images, acquired by ITV News and broadcast Sunday, appeared 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. Both are being questioned in a south London police station.

One was arrested in the ferry departure area in the port of Dover on the English Channel. The other was arrested in west London. Both are being held under the Terrorism Act.

Police have searched several properties, including a home in Sunbury where an elderly couple served as foster parents for many children over several decades. They have been honored by Queen Elizabeth II for their work on behalf of children.

They have recently helped refugees from conflict zones including Syria and Iraq.

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.