A passenger walks onto the platform at Parsons Green subway station after it was reopened, Sept. 16, 2017, following a terrorist attack on a train at the station in London.
A passenger walks onto the platform at Parsons Green subway station after it was reopened, Sept. 16, 2017, following a terrorist attack on a train at the station in London.

British police say that they have arrested a second suspect in connection with the bombing of a subway train in London.

The police said in a statement they arrested a 21-year-old man in the west London suburb of Hounslow, which is home to London’s Heathrow Airport, just before midnight Saturday. He was arrested under Britain’s Terrorism Act.

Earlier Saturday, another suspect, an 18-year-old man was arrested in the port area of Dover, a major ferry terminal for travel between Britain and France.

“He was arrested on the suspicion of being concerned in the commission, preparation or instigation of acts of terrorism,” Deputy Assistant Police Commissioner Neil Basu said in a Saturday news conference.

WATCH: Scotland Yard Official: 121 Witnesses to Attack Identified

Basu said the first arrest was significant. Following that arrest, police evacuated the Dover port and a suburban London neighborhood as they searched a nearby house.

Residents of the Sunbury neighborhood say the house that was searched is occupied by an elderly couple, Penelope and Ronald Jones, who have taken care of foster children for decades. Queen Elizabeth honored them for their efforts in 2010.

Basu said a “number of items” were recovered from the Dover terminal, without giving further details.

WATCH: Police Manhunt Following Terror Attack on London Underground

“We are still pursuing numerous lines of inquiry, and at a great pace,” Basu said. “Our priorities ... are to identify and locate any other suspects,” he added.

Earlier Saturday London Transport authorities said they have re-opened the Parsons Green station where the bomb on a train partially detonated.

Images of the bomb posted on social media appear to show a bucket on fire that had been placed inside a plastic bag close to a railcar door.

map of parsons green subway in london britain
Map of Parsons Green subway in London

Prime Minister Theresa May said after the attack that the country’s Joint Terrorism Analysis Center decided to raise the country’s threat level to “critical,” meaning that a further attack may be imminent.

London police commissioner Cressida Dick told reporters Saturday the threat level has been altered.

“We have a very considerable threat. My colleagues in the intelligence agency would say that this is a shift in threat, it’s not a spike. I won’t go on in detail about that, but it is a change threat,” Dick said.

WATCH: Scotland Yard Official: 'Threat Level Remains at Critical'

ISIS has claimed responsibility for the assault. The terrorist group, however, has a history of claiming responsibility for attacks the group may not be connected with.

May said the public may see more armed police on the streets and the transport network. The prime minister also said members of the military will begin aiding police, providing security at some sites not accessible to the public.

The blast was the fifth major terrorist attack in Britain this year.

London police said their investigation into Friday’s attack is being supported by MI-5, Britain’s domestic intelligence agency.

London Mayor Sadiq Khan said the British capital “will never be intimidated or defeated by terrorism.”