Police have been granted more time to question two suspects in connection with Friday's Tube attack.
On Saturday an 18-year-old man, thought to be an Iraqi orphan, was arrested in Dover port, and a 21-year-old was detained in Hounslow.
A homemade bomb partially exploded in a train at Parsons Green station, injuring 30 people in rush hour.
Police now have until 23 September to question the 18-year-old, and until 21 September to detain the 21-year-old.
The 18-year-old is a recent foster child at a home in Sunbury-on-Thames, and the 21-year-old has links to the property, where a police cordon is in place.
Police are searching the house owned by foster parents Ronald Jones, 88, and Penelope Jones, 71.
Both men were arrested under section 41 of the Terrorism Act and were granted warrants for further detention by Westminster Magistrates' Court.
They remain in custody at a south London police station.
CCTV images have emerged showing a person carrying a Lidl supermarket bag 90 minutes before the bombing.
BBC News has obtained CCTV footage showing a person leaving the property in Sunbury carrying a Lidl bag at 07:04 BST on Friday morning.
At 08.20, a device exploded on a District Line train.
The bomb was in a plastic bucket, which was transported in a Lidl bag.
The 18-year-old is thought to have moved to the UK aged 15 when his parents died.
Neighbours claimed the teenager, who is thought to still live at the foster home, tried to run away.
Neighbours who spoke to the BBC reported police cars attending the address several times over the last few months.
Resident Dave Solway said: "If they don't come back on time the police are called, that's why there will be a police presence.
"Whether that means the lads will cause trouble per se, who's to know? That's speculation."
A 21-year-old man, believed to be Syrian-born Yahyah Farroukh, was also arrested on Saturday in Hounslow, west London, and his house in Stanwell, near Heathrow airport, is being searched.
Mr Farroukh was a former foster child who had lived in the Jones's house in Sunbury, local council leader Ian Harvey said.
Mr Farroukh posted a picture on his Instagram page in May this year with a suitcase on Cavendish Road, Sunbury, almost directly outside the Jones's house.
Mr and Mrs Jones were made MBEs for services to children in 2010, and friend Alison Griffiths said the couple had an 18-year-old and a 22-year-old staying with them recently.
She said the couple were "great pillars of the community", adding: "They do a job that not many people do."
Map of events
Mr Farroukh is believed to have been in the UK for at least four years and learned English at West Thames College between 2013 and 2015.
Neighbours told the BBC he was a "friendly" and "chatty" young man who had been visited by family from Scotland.
Police also searched the Aladdins chicken shop in Hounslow, west London, where Mr Farroukh was arrested. A manager confirmed he had worked at the Kingsley Road shop for a number of months.
Shop owner Suleman Sarwar said police searched lockers on the premises. He added: "We didn't know him very well.
"He was quite quiet, he just worked over here and that was it.
"He was very normal. If he did do anything you would never have suspected it."
The so-called Islamic State group claimed responsibility for the attack.
Home Secretary Amber Rudd said while there was "no evidence" to confirm Islamic State's involvement, work was being done to find out how the attacker was radicalised.
The UK's terror threat level was raised to critical - its highest level - immediately after the attack, but it was brought back to severe after the police arrests.
Assistant Metropolitan Police Commissioner Mark Rowley said police had gained a "greater understanding" of how the bomb was prepared but said there was "still much more to do".
Thirty people were injured - most suffering from "flash burns" - when a bomb was detonated on a Tube carriage at Parsons Green station.
Jack Durston was on the train where the bomb went off on Friday and he said: "There was a big sort of wall of fire, the whole width of the carriage, moving towards us very fast.
"My instinct was to just run. And I ran away from the fire and out the door."
Police have urged anyone with information to contact them and to upload pictures and video to the website www.ukpoliceimageappeal.co.uk or to call the Anti-Terrorist Hotline on 0800 789 321.
Did you witness the arrest in Hounslow? Share your pictures, video and experiences by emailing firstname.lastname@example.org.
Please include a contact number if you are willing to speak to a BBC journalist. You can also contact us in the following ways: