London ? Police swooped down on a posh London neighborhood and traced cell phone calls across Europe to a Rome hide-out Friday, netting the remaining suspects in the failed transit bombings without firing a shot. The arrests capped an eight-day manhunt that was one of the most extensive in British history.

At least three of the four suspects were of East African origin.

Black-clad police armed with stun grenades and gas masks pointed assault rifles at the doors of suspects on the outskirts of Notting Hill. Two young children stumbled into the standoff a floor below a suspects’ apartment, and an armed officer tried to shoo them away from his dog.

Above them, a police team shouting for “Mohammed” forced two suspects to strip to their underwear and eventually emerge onto a narrow balcony, where television cameras recorded them with their hands above their heads.

Police officers dressed in white overalls cover the face of a suspect, centre, as he is led from an apartment building in the Notting Hill area of London, Friday in this image made from television. Heavily armed police wearing gas masks and reportedly using stun grenades raided the apartment block seeking suspects in the failed July 21 attacks on London's transport system.

In Rome, police arrested a Somali-born British citizen at the apartment of his brother, who was also taken into custody. On Friday night, a police expert – wearing white gloves and a jumpsuit to avoid contaminating possible evidence – could be seen working inside a lighted room in the apartment.

Images captured on closed-circuit television cameras during the failed July 21 attacks helped lead investigators to the men, and interrogations of the suspect captured first, 24-year-old Yasin Hassan Omar, may have helped as well. Police said the anti-terrorist sweeps have been part of their most extensive investigation ever.

Peter Clarke, head of Scotland Yard’s anti-terrorist branch, sounded a cautionary note as he announced the arrests.

“Despite the progress that has been made with the investigation, we must not be complacent,” Clarke warned. “The threat remains, and is very real.”

Authorities have been looking for a link between the failed July 21 attacks and the July 7 suicide bombings, which killed 56 people, including the four bombers. Three of the suicide attackers had links to Pakistan.

They have a working hypothesis that the cells did not know each other, but were connected by a senior operative higher up an organizational chain, said a counterterrorism official, who spoke on condition of anonymity because investigations are ongoing.

Authorities also are theorizing the plot may be linked back to Pakistan and the al-Qaida organization there. However, “we are not there yet in terms of proving it,” the official said.