Updated at 4 a.m. ET Sunday
Police in the U.K. have now arrested two men in connection with the explosion on a train Friday that left dozens injured.
London's Metropolitan Police announced on Sunday morning that they arrested a 21-year-old man in the west London area of Hounslow late on Saturday night.
Earlier on Saturday police made what they said was a "significant arrest" of an 18-year-old man in relation to the investigation.
Metropolitan Police said Kent Police arrested the first man Saturday morning local time in Dover, about 75 miles southeast of London on the country's coast.
They said he was in the departures area of the Port of Dover; the port is the main ferry link to France. Police also searched the port and said they "recovered a number of items" during the search.
Police also raided a house in the town of Sunbury-on-Thames Saturday afternoon, which is located to the southwest of London.
An improvised explosive device went off around 8:20 a.m. Friday on a train at the Parsons Green subway station, according to authorities. Police now say 30 people "are known to have been injured" in what they call a terrorist attack. Reports describe victims with facial burns.
Metropolitan Police released a statement from Deputy Assistant Commissioner Neil Basu, a counterterrorism coordinator.
"We have made a significant arrest in our investigation this morning. Although we are pleased with the progress made, this investigation continues and the threat level remains at critical.
"The public should remain vigilant as our staff, officers and partners continue to work through this complex investigation. We are not, at this time, changing our protective security measures and the steps taken to free up extra armed officers remain in place.
"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."
Both men are being held in police custody in London.
The Islamic State claimed responsibility for the attack, though that could mean different things, as The Two-Way pointed out earlier this year.
The U.K. has suffered at least five attacks since March, including two bridge attacks with vehicles and knives in London, a vehicle attack outside a London mosque and the bombing at the Ariana Grande concert in Manchester that killed 22 people and injured dozens.