Scotland Yard has issued CCTV images of four men it wants to question over the failed bomb attacks on three Tube trains and a bus in London.
It is not yet clear if a man shot dead by plain clothes officers at Stockwell Tube station on Friday was one of them.
Scotland Yard urged anyone who knew the whereabouts of the men captured on CCTV to call 999.
They said if they could identify any of them they should call the anti-terrorist hotline on 0800 789 321.
Members of the public should not approach the men under any circumstances, police warned.
After Friday's shooting another man was arrested at an address in Stockwell, south London. Officers then searched the property.
The man was held on suspicion of the preparation, instigation and commission of acts of terrorism.
He was taken for questioning to Paddington Green Police Station in central London.
Officers raided several addresses across the capital on Friday, including one in Harrow Road, north-west London, where people reported bangs thought to have been the firing of CS gas canisters.
Also on Friday, a 29-year-old West Yorkshire man who had been held since 12 July, on suspicion of the commission, instigation or preparation of acts of terrorism, was released without charge.
He had been arrested on the same day as raids were carried out in the Leeds area in connection with the original bombings on 7 July.
Metropolitan Police Commissioner Sir Ian Blair told a news conference Thursday's attacks and those two weeks earlier had left Scotland Yard facing "the greatest operational challenge" in its history.
He said: "Officers are facing previously unknown threats and great danger
"This operation is targeted against criminals - not any community or section of a community."
Thursday's attacks began at about 1230 BST, with bombs at Warren Street station, central London, Shepherd's Bush station in the west, Oval in the south and on a bus in Shoreditch, east London.
Along with the CCTV images, police revealed details about the suspects' movements:
- The first image showed a man in a black sweater running away from Oval station's Northern Line at 1234 BST on Thursday. Police believe he had travelled north from Stockwell. His top was later found in nearby Brixton.
- The second image showed a man on the number 26 bus travelling from Waterloo to Hackney Wick. He got off the bus at Hackney Road at about 1306 BST.
- The third image showed a man leaving Warren Street station at 1239 BST.
- Police believe the man pictured at Westbourne Park station at 1221 BST travelled west on the Hammersmith and City Line to Shepherd's Bush, where he ran from the station.
Following the incidents, streets were cordoned off, parts of the transport network closed and stations evacuated, but no-one was badly hurt.
Three of the devices found were the same size and weight as those used for the 7 July London bombings, while the fourth was smaller and appeared to have been contained in a plastic box. The same chemicals appear to have been used.
Scotland Yard Assistant Commissioner Andy Hayman told the news conference: "At this stage it is believed the devices consisted of homemade explosives and were contained in dark coloured bags or rucksacks. It is too early to tell how these were detonated."
BBC security correspondent Gordon Corera told BBC Radio 4's Today programme the devices were so similar there was speculation they could have been part of the same batch.
"The explosive might have degraded over time or had not been put together right in this case, or it could have been a completely different batch of explosives - homemade - that had not been cooked up properly."
The bombers' plan might have been disrupted by the investigation into the 7 July attacks, forcing them to act before they were fully prepared, Mr Corera added.
Police have asked that any images of the attacks are sent to www.police.uk. The hotline number for anybody with information is 0800 789 321. Witness reception points have been set up near the four scenes.