LONDON, AP -London police have three suspects in custody after last week”s failed terrorist strikes but say they are looking for many more because investigators now believe a wide network of al-Qaida-linked operatives staged the attacks.

The network they are trying to roll up could include bomb makers and those who coached the young suicide attackers before their mission, according to police.

Investigators were also pursuing leads that seemed to indicate a link between the unsuccessful attacks on July 21 — when bombs only partially detonated — and the suicide bombings two weeks earlier which killed the four bombers and 52 other people.

Metropolitan Police Commissioner Sir Ian Blair said he believed al-Qaida terrorists were involved in both attacks.

&#34The way in which al-Qaida operates is not a sort of classic cell structure,&#34 Blair told Britain”s Sky News television on Sunday. &#34It has facilitators, so we”re looking for the bomb makers, we”re looking for the chemists, we”re looking for the financiers, we”re looking for the people who groomed these young people, so it will be a wide network that we”re trying to penetrate.&#34

Asked if the two attacks were connected, Blair said &#34we have no proof that they are linked, but clearly there is a pattern here.&#34

Two men were arrested in London”s southern Stockwell neighborhood on Friday under Britain”s anti-terrorism laws, the Terrorism Act 2000. A third man was arrested under the same legislation in nearby Tulse Hill on Saturday. All three are being held &#34on suspicion of the commission, instigation or preparation of acts of terrorism&#34 for the unsuccessful, July 21 attacks.

Blair said police were &#34still anxious for any sighting of the four individuals&#34 whose images were released after being spotted on surveillance camera footage fleeing the scenes of the failed bombings, suggesting that none of the four are among the men now in custody .

Police were reportedly investigating whether some of the July 21 suspects may have visited the same Welsh whitewater rafting center as two of the July 7 suicide bombers: Mohammed Sidique Khan and Shahzad Tanweer.

The two bombers went whitewater rafting there on July 4, according to the National Whitewater Center. Police have refused to comment on reports that a brochure for the rafting center was found in an explosives-laden knapsack that failed to detonate on a bus on July 21.

The fast-moving investigation into the attacks has been marred by the death of a Brazilian electrician, Jean Charles de Menezes, who was shot and killed Friday by police officers who mistook him for a suicide bomber.

Menezes was followed by plainclothes officers after he left an apartment bloc in Tulse Hill that was under surveillance. Wearing a padded jacket, he boarded a bus and traveled to the nearby Stockwell subway station. According to officials, his clothing and behavior aroused the suspicions of the police who ordered him to stop.

Witnesses said Menezes ran into a subway car, where officers shot him. It was unclear why Menezes, who spoke English, did not stop.

Commissioner Blair expressed deep regret for Menezes” killing, which he described as a &#34tragedy,&#34 but defended his officers” right to use deadly force against suspected suicide bombers.

&#34I am very aware that minority communities are talking about a shoot-to-kill policy; it”s only a shoot-to-kill-in-order-to-protect policy,&#34 he said. &#34The only way to deal with this is to shoot to the head … There is no point in shooting at someone”s chest because that is where the bomb is likely to be.&#34

In emotionally charged visits, relatives and friends of people killed in the July 7 explosions visited three subway stations, and a central London street where a double-decker bus was blown up, to pay their respects. Many broke down in tears as they laid flowers at the sites of the explosions. Earlier, more than 230 relatives and friends attended a briefing on the investigation by police.