LONDON (AP) -- British police made a "significant" arrest Saturday in the manhunt for suspects a day after the London subway was hit by a partially-exploded bomb and launched a heavily armed search of a home southwest of London.
The fast-moving inquiry into the subway blast that wounded 29 people has shifted to Sunbury, on the outskirts of the British capital, where neighbors were evacuated amid the police operation as a precaution.
A no-fly zone was established over the area to keep out small planes and drones as police moved in and police cordons were put in place to keep the public well away.
No details about the police search were released, but it came after the arrest of an 18-year-old man who is being held under the Terrorism Act. The man was arrested Saturday morning by Kent police in the port of Dover on the English Channel.
Dover is a major ferry port for travel between Britain and France - and it was not clear if the suspect was trying to board a ferry for France when he was taken into custody.
"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 that top British security services believe that another attack is imminent.
Basu's comments suggested that other dangerous suspects may still be at large.
Police Commissioner Cressida Dick called the arrest "very significant" but said the public should still be vigilant.
The 18-year-old suspect hasn't been charged or identified. Police say he was being brought to a south London police station for more questioning. Police haven't said if he is suspected of planting the bomb or of playing a supporting role in a possible plot.
Authorities had increased Britain's terrorism threat level to "critical" late Friday - the highest possible level - after a bomb partially exploded on a subway train during the morning rush hour.
Police are combing through closed-circuit TV images and have extensively studied the remains of the explosive device. Images from inside the subway car showed that it was contained in a bucket with wires hanging out of it and concealed in a plastic shopping bag.
The train hit by the bomber at Parsons Green station in southwest London had video cameras in each car, and the London Underground network has thousands of cameras at the entrances to stations and along its labyrinth of subterranean and aboveground passageways.
Officials have hinted there may be more than one person involved, but haven't released details in what is termed an ongoing and covert inquiry.
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 further attacks.
The bomb went off around 8:20 a.m. Friday as the District Line train, carrying commuters from the suburbs - including many school children - was at the Parsons Green station. In all, 29 people were wounded, some with burns, but none of the injuries were believed to be life-threatening.
The station was reopened Saturday, officials said, restoring some normalcy to London's transport network after a day of severe disruption. There was no sign of panic among Londoners and the weekend life of the multicultural city continued undeterred by the raised threat level.
Officials said the bomb was intended to do grave harm to commuters. Analysts said the carnage would have been far worse had the entire device exploded.
"They were really lucky with this one. It could have really become much worse," said terrorism specialist Magnus Ranstorp of the Swedish Defense University.
The Islamic State group claimed responsibility for the attack, which it said was carried out by an affiliated unit.
Britain has endured four other attacks this year, which have killed a total of 36 people. The other attacks in London - near Parliament, on London Bridge and near a mosque in Finsbury Park in north London - used vehicles and knives.
In addition, a suicide bomber struck a packed concert hall in Manchester in northern England, killing 22 people. That attack in May also briefly caused the threat level to be set at "critical."