LONDON -- Two of the six men arrested over last week's London subway bombing were released without charge Thursday, British police said. Four others remained in custody.

One of those released is a 21-year-old, originally from Syria, detained outside a fast-food restaurant in west London on Saturday, a day after the rush hour attack on a subway train. The other is a 48-year-old man arrested Wednesday in Newport, Wales.

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.

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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 were charged, and their names weren't released.

Authorities reduced the terror threat level from "critical" to "severe" in the days following the attack, indicating they view another attack as highly likely -- but no longer imminent. Police asked the traveling public to be vigilant and to expect to see an increase in armed police.

Most of the injured in Friday's explosion aboard a subway 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 of Iraq and Syria (ISIS) has claimed responsibility for the attack, but British officials say there is no proof yet that it was involved.

This was the fifth attack in Britain in the last 6 months.