The homicide bombers who struck London's transit network did reconnaissance ahead of time and used peroxide-based explosives that took skill to assemble, suggesting the deadly attacks were carefully planned, police said Tuesday.

Closed-circuit television footage taken June 28 shows three of the four bombers following the same route they took on the day of the bombings, which killed 56 people including themselves.

"What this told us about these people is that this was carefully planned," said Peter Clarke, head of the police anti-terrorist branch. "This is quite typical of terrorists' methods ... to reconnoiter the layout, the timings, to check security, to check all those things that they would feel in their mind they needed to get right."

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The three men wore T-shirts and baseball caps. Two wore smaller backpacks than the ones believed to have carried the deadly July 7 bombs. A fourth, alleged bus bomber Hasib Hussain (search), was not present.

The pictures show the three bombers entering Luton train station north of the capital, arriving at London's King's Cross and entering the Underground system (search). Footage was also found showing the three at the Baker Street subway station, close to one of the bombing sites.

The images bore striking similarities to television pictures released earlier showing the suspects entering Luton station on July 7 and walking inside, with heavy knapsacks strapped to their backs.

Detectives located the June 28 images after finding train tickets and receipts.

Britain's closed-circuit cameras are owned by many entities — including private transit companies, traffic monitors, shops and banks — so there is no rule on how long tape must be stored. Police seized about 80,000 recordings after the bombings and are reviewing them.

The suspects — Mohammad Sidique Khan (search), Shahzad Tanweer (search) and Germaine Lindsay (search) — spent about three hours in London on June 28 and may have split up during that time, police said. Investigators were still searching closed-circuit tapes for more about what the bombers did in the city.

Clarke also gave the first official confirmation that the bombs were based on peroxide, as the New York police said in August without authorization from London.

He said one of the apartments police had searched in Leeds, the northern English city that was home to three of the four bombers, had been a "carefully thought out and effective (bomb) manufacturing facility."

The attackers carried explosives in coolers the morning they struck, he said.

Detectives found two unexploded bombs in a Nissan Micra they left at Luton station, Clarke said. Those devices were also made of peroxide and were surrounded by nails, he said.

Clarke said police also found traces of HMTP, a type of peroxide explosive, in the car.

Andy Oppenheimer, an explosives expert with Jane's Information Group, said HMTP was one of a group of explosives made from common, easily available items such as hydrogen peroxide, which can be used as a hair bleach or antiseptic. The ingredients are tricky to assemble and can blow up accidentally in unskilled hands, he said.

Assistant Police Commissioner Andy Hayman said the investigation was "a colossal undertaking."

Among the 15 sites detectives have examined is a landfill in West Yorkshire, where forensic searches are still going on, Clarke said.

Police said they have interviewed more than 3,000 witnesses, and seized nearly 30,000 items from the sites, which included homes linked to the bombers in and near Leeds.

More than 1,000 of the seized items came from the apartment where police believe the bombs were made, they said. The search of that home took more than six weeks and ended only recently, the police said.

The fresh details came after Al Qaeda deputy Ayman al-Zawahri (search) claimed responsibility for the attacks in a statement broadcast Monday on an Arab television station.

Clarke appealed for information from people who might have seen the bombers. He also asked for information on people the suspects might have known. Investigators have said they believe co-conspirators helped plan the attacks, but no one is in custody.

"We really need to know, did they meet anyone else? What else did they do during that time?" Clarke said of the three hours the bombers spent in London on June 28. "Those missing three hours could be vital."