UK lowers terror threat level as London subway bomb probe advances

London: Police stepped up their investigation Monday into the bombing of a packed London Underground train during rush hour after officers made a second arrest in their probe.

The bomb went off on Friday's morning in a crowded carriage and although the device is thought to have malfunctioned, it still wounded 30 people. Britain downgraded on Sunday the nation's terrorism threat from its highest level following the arrest.

It was the country's fifth terror attack in six months, a series that has claimed 35 lives.

Police said earlier Sunday that a 21-year-old man, who has not been identified, was detained late Saturday in Hounslow, on the western rim of the capital.

"The Joint Terrorist Analysis Centre, which reviews the threat level that the UK is under, have decided to lower that level from critical to severe," interior minister Amber Rudd said in a televised statement.

A critical threat level means another attack is "expected imminently" while a severe threat indicates an assault is highly likely.

A search was underway on Sunday in Stanwell, a few miles west of Hounslow, in connection with the 21-year-old's arrest, police said.

After taking into custody an 18-year-old man earlier on Saturday over the "bucket bomb" attack at the Parsons Green Underground station, police said they they were hunting for more suspects.

Rudd said the police were trying to find out how the first man arrested was "radicalised".

The Islamic State group claimed responsibility for Friday's explosion.

The first arrest on Saturday took place at the Dover ferry terminal, a main link to Europe. A "number of items" were recovered during the operation and the man is now in custody in London, officers said.

Police also raided a home in Sunbury, a town west of London on Saturday. Local residents quoted in British media said the owners of the house were elderly foster parents.

CCTV footage obtained by British media on Sunday appeared to show a man walking from the property on Friday morning, carrying a bag similar to the one containing the failed device.

- Passengers flee in panic -
Now that the terror threat has been downgraded, soldiers deployed to guard key sites across the country "will return to their original positions" during the next few days, Rudd said.

The critical warning had last been used after the deadly suicide bombing at a pop concert in the northwestern city of Manchester in May, for which IS also claimed responsibility.

But Rudd voiced doubt over the IS claim that it was behind Friday's bombing.

"It is inevitable that so-called Islamic State or Daesh will try to claim responsibility but we have no evidence to suggest that yet," she told the BBC.

Rudd had earlier dismissed as "pure speculation" US President Donald Trump's claim, made Friday on Twitter, that a "loser terrorist" behind the attack was known to Scotland Yard.

The tweet had already drawn a terse rebuke from Prime Minister Theresa May, who said: "I never think it's helpful for anybody to speculate on what is an ongoing investigation."

In another security scare on Sunday, a London-bound British Airways flight was evacuated at Paris's Charles de Gaulle airport after a false bomb alert.

The improvised device at Parsons Green, a quiet and well-off residential district, failed to detonate fully.

But the blast inflicted flash burns on passengers, and prompted dozens of others to flee in panic.

- 'Fireball flew down carriage' -
Twitter user @Rrigs posted pictures of a white bucket smouldering on the train and described how a "fireball flew down carriage and we just jumped out open door".

The bucket, which was inside a frozen food bag, looked like the type used by builders, and there appeared to be wires coming from it.

Louis Hather, 21, had been travelling to work and was three carriages down from where the explosion took place.

"I could smell the burning. Like when you burn plastic," he told AFP.

He was trampled on as panicking passengers stampeded out of the station, leaving him with a bad cut and bruised leg.

The bomb's remnants were examined by forensic scientists but no further details were released.

Several victims were taken to hospital, though health authorities said none were in a life-threatening condition.