A day-long manhunt after the terror attack near London has netted one arrest — but police hint that they’re still looking for more potential suspects. Police captured an 18-year-old man in the transit port of Dover earlier today after the bombing of a train in Parsons Green that apparently fizzled, while still injuring more than two dozen commuters. ISIS has claimed credit for the attack, but thus far police have not confirmed that aspect of the incident:

Police arrested an 18-year-old man Saturday morning in the port area of Dover in connection with an attack a day earlier on a train at a London Underground station, London’s Metropolitan Police said.

It was a “significant arrest” as the investigation continues, Deputy Assistant Commissioner Neil Basu, the senior national coordinator for counterterrorism policing, said in a statement.

“This arrest will lead to more activity from our officers. For strong investigative reasons, we will not give any more details on the man we arrested at this stage,” he said.

“More activity” suggests that police suspect a wider conspiracy rather than a lone-wolf attack. That may be from an abundance of caution rather than having evidence of a terror cell at work. The incompetence of the execution of the attack, plus the lack of follow-up attack, would normally point to a single perpetrator. At this stage it’s all mostly just speculation, but a more coordinated ISIS- or al-Qaeda-backed attack would usually have multiple events designed to stretch security resources thin enough to facilitate escape, or at least maximum casualties.

Nevertheless, London police are taking no chances on follow-up attacks today. They have deployed soldiers to reinforce local police forces in securing the trains today:

“We have made a significant arrest in our investigation this morning,” Deputy Assistant Police Commissioner Neil Basu said. But he warned that the investigation was ongoing and the terrorist threat level remains at “critical,” meaning a government task force that includes the security services believes another attack is imminent. …

Prime Minister Theresa May said raising the threat level to its highest point was a “proportionate and sensible step.” Police called on the public to be vigilant.

The soldiers will add to the armed police presence Saturday at public places to deter attacks after the Friday morning rush-hour blast on a District Line train.

They’re not just bolstering security around the trains, either. The UK announced that heightened security measures would be implemented throughout the country. As for the ISIS claim, one official reminded viewers that they have a habit of making claims regardless of actual connections:

On the other hand, London has become the favorite target for radical Islamist jihadis this year, so an ISIS claim of responsibility isn’t too difficult to believe. CNN lists five terror attacks in 2017 alone, which should have the UK asking questions about what makes their capital so attractive for these kinds of attacks, and the changes necessary to reset the incentives.

Update: Shy News has live updates here on a massive police raid in Surrey believed to be linked to the investigation. A number of residents were told to evacuate immediately:

Cavendish Road closed here in Sunbury as police raid property linked to #ParsonsGreen pic.twitter.com/4V6q92O7Bs

— Dan Whitehead (@danwnews) September 16, 2017

The mother-of-three told the Press Association: “I was in my house with my children and there was a knock at the door from the police.

“They told me to leave. They said: ‘You have one minute to get out of the house and get away.’ …

“Lots of residents are out and about .. many of them have been told to leave their houses this afternoon.

“They haven’t been allowed back and they are wondering what to do next. One woman wants to get her medicines back from her house, but she’s not being allowed to do so. They don’t have much information when they will be allowed back in.”