Armed police officers walk through a carriage of a London underground tube train at Parsons Green station after the attack.

:: A second man has been arrested by police investigating Friday’s bomb attack on a London underground train, police said.

The 21-year-old man was arrested in the west London suburb of Hounslow just before midnight on Saturday, the Metropolitan Police force said in a statement.

He was detained under Britain’s Terrorism Act and taken to a south London police station, the force added on Sunday.

On Saturday, police arrested an 18-year-old man in connection with the attack.

Friday morning’s blast, which left 29 people injured, was Britain’s fifth terror attack in six months and was claimed by ISIS.

Prime Minister Theresa May announced late Friday that the threat level had been raised to “critical”, and said troops would take over guarding key sites to free police officers for deployment elsewhere.

The Parsons Green Tube station may not have been the intended target for the terror attack, security experts have told British media.

The homemade bomb which exploded on the District Line Underground train may actually have been intended for a bigger, busier station for ‘maximum impact’.

Will Geddes, founder of private security firm International Corporate Protection, believes major stations such as Paddington or Westminster may have been the real target.

The explosion at Parsons Green station in south-west London sparked a “wall of fire” that left passengers with burns and caused a stampede of panicking people in which some were trampled.

The anti-terrorism police chief Mark Rowley said the investigation was making “really good progress”.

“We’re chasing down suspects,” he told reporters.

“Somebody has planted this improvised explosive device on the Tube. We have to be open-minded at this stage about him and potential associates.”

May said military personnel would take over guard duty at certain closed “protected sites”, freeing up 1,000 police officers to be deployed on the transport network and on streets across Britain.

The country was last on critical alert after the bombing at a concert in Manchester in May, which was also claimed by ISIS.

In a statement on Friday, the group said an ISIS “detachment” had carried out the London attack.

Rowley earlier said most of the injuries were due to “flash burns”, while others were wounded as passengers ran out of the station in panic.

In this image made from video, fire raises at a southwest London subway station in London Friday, Sept. 15, 2017.