Anti-terror police in the United Kingdom are continuing to question suspects recently arrested on suspicion of attempting to blow up a London Underground train.
Syrian refugee Yahya Faroukh, 21, and an 18-year-old believed to be from Iraq, remain in police custody.
The uk's top counter-terrorism officer, Assistant Commissioner Mark Rowley, said police were gaining a "greater understanding of the preparation of the device".
'A significant amount of activity has taken place since the attack on Friday. "Detectives are carrying out extensive inquiries to determine the full facts behind the attack", Cmdr.
Police said Thursday that terrorism-related arrests had risen 68 percent over the past year.
"Searches are continuing at two addresses in Surrey and a commercial property in Hounslow in connection with this ongoing investigation".
Both men are being held for questioning under the Terrorism Act.
Earlier in the day, the Metropolitan police said it had arrested a second suspect believed to be linked to the blast that went off in a carriage at Parsons Green on Friday. Farroukh was the second person to be arrested.
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Officials stopped Farroukh outside a fried chicken restaurant in the Hounslow area of west London on Saturday night.
An unnamed witness came forward to say: "The guy had just walked past the takeaway when three blokes and a woman came running past and he was rugby-tackled to the floor".
Other police waiting in cars in surrounding streets grabbed his mobile phone, put plastic bags over his hands, feet and legs and forensics teams took pictures of him and the scene.
He added: 'As police and our partners do all we can to confront the threat, public vigilance remains as vital as ever.
A local politician said he understood the 18-year-old suspect to be an Iraqi orphan who had moved to Britain aged 15 after his parents died.
Queen Elizabeth II honored Ronald and Penelope Jones for their fostering work in 2010, awarding them MBEs (Member of the Order of the British Empire) "for services to children and families".
But when asked if ISIS may have had a hand in the attack, UK Home Secretary Amber Rudd said: "It is inevitable that so-called Islamic State or Daesh will reach in and try and claim responsibility".
Anyone with information is urged to call the Anti-Terrorist Hotline on 0800 789 321 or, in an emergency, always call 999.
He said the 21-year-old was also a former foster child of the Joneses.