Police officers walk behind cordon tape set up around a property being searched in Sunbury-on-Thames on Sept. 17. (Peter Nicholls/Reuters)

Following a fast-moving investigation and manhunt, British police on Saturday arrested a second man in connection with a detonation on the London subway during the Friday morning rush hour, in which at least 30 people were injured. Police labeled the attack terrorism.

The man, 21, was arrested just before midnight Saturday in West London. Police did not announce his detention until Sunday morning.

Earlier, an 18-year-old man had been arrested by Kent police in the port area of Dover on the English Channel. Police suspect he might have been seeking a boat out of England.

In addition, armed police raided and searched a house in Sunbury, west of London, on Saturday afternoon. Counterterrorism units were at the scene, and police told reporters the operation was connected to the subway explosion.

A homemade bomb exploded on a London subway train at Parsons Green station Friday morning, sending a scorching blast of flame and smoke through a London subway car.

Home Secretary Amber Rudd told the BBC on Sunday that the second arrest suggested the attack was not the work of a “lone wolf” terrorist and implied some level of conspiracy.

On Saturday, Rudd said Saturday that it was “good fortune” the improvised explosive device “did so little damage,” but she said that the materials used to build the bomb were too readily available.

“We have to make certain we take all the steps we can to ensure that the sort of materials this man was able to collect become more and more difficult to combine together,” she said. 

Deputy Assistant Police Commissioner Neil Basu called the arrest of the teen at the Dover port “significant” and said the investigation is ongoing. 

Both men are being held for questioning under the Terrorism Act. “For strong investigative reasons, we will not give any more details on the man we arrested at this stage,” Basu said, speaking of the teen.

In the town of Sunbury-on-Thames, located about 15 miles to the west of central London, residents waited outside a police cordon on Saturday evening, as forensics experts entered a rowhouse on Cavendish Road.

Anna Wilkins 43, lives next to the house being searched. “I saw a young man come out of there with his bike a couple of times in recent weeks,” Wilkins said. The young man, whom she described as “Asian,” arrived at the house just a couple of months ago and lived with an elderly couple. It is unknown whether the young man described by Wilkins is the suspect arrested in Dover.

“I never spoke to him and only saw him when he left the house with his bike, but I was always suspicious of him,” Wilkins said.

The BBC later reported that the couple who owns and lives in the house that was searched, Ronald and Penelope Jones, were respected members of the community who had worked as foster parents for “hundreds of children, including refugees.” They were awarded for their service with honors from Queen Elizabeth II in 2010.

One resident living near the house being searched said he had never seen anyone entering or leaving it. “This isn’t an area where people really know each other,” said Chris Ross, 51. “This afternoon, there were suddenly armed police officers who told us to get out of our houses as soon as possible. They only gave us a couple of seconds.”

After the bombing, security measures were immediately tightened across London’s vast mass-transit network, and the government described the threat level as critical, meaning another attack could be imminent.

The Islamic State terrorist group asserted responsibility for the explosion. Experts cautioned that the group often seeks credit for attacks it may have only inspired, as well as ones it had nothing to do with.

Metropolitan Police Commissioner Cressida Dick traveled — escorted by journalists — by subway to Waterloo station and “patrolled” the South Bank of the Thames.

“Yesterday we saw a cowardly and indiscriminate attack, which could have resulted in many lives being lost,” Dick said. “Since then, we have had teams of detectives and specialists working through the night on the investigation, and officers throughout London mobilizing and providing an increased visible police presence — especially in crowded places.”

The explosion on London’s Tube rekindled debate about whether countries such as Britain have been tough enough in fighting terrorism. Just hours after the blast, President Trump suggested that Britain needed to be “more proactive.” Prime Minister Theresa May retorted that such comments were not helpful.