LONDON — British police arrested a third man in connection with last week’s failed attack against London’s transit system and said yesterday they were trying to penetrate what they suspect is an Al Qaeda network behind the plot.

Police Commissioner Sir Ian Blair expressed deep regret to the family of a Brazilian electrician shot dead by police on the subway Friday after he was mistaken for a terrorist, Jean Charles de Menezes. Mr. Blair called the killing a “tragedy,” but defended officers’ right to use deadly force against suspected terrorists.

The latest arrest was made Saturday in an area near London’s southern Stockwell neighborhood, Tulse Hill, where Menezes had lived and near the subway station where he was killed. The man was arrested “on suspicion of the commission, instigation, or preparation of acts of terrorism,” a police spokeswoman said on customary condition of anonymity.

Police also said they carried out several controlled explosions yesterday to destroy a package found in northwest London that may have been linked to devices used in the botched attacks.

Mr. Blair said he suspected an Al Qaeda network was involved in last Thursday’s failed attacks. He had previously said Al Qaeda was probably linked to the July 7 attacks as well, in which four suicide bombers killed 52 people and themselves.

“The way in which Al Qaeda operates is not a sort of classic cell structure,” the police chief told Britain’s Sky News television. “It 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.”

Attorney General Gonzales said yesterday the recent attacks in London and Egypt appear to be the work of Al Qaeda.

Police were looking into possible links between the bombers who took part in the July 7 attacks against three subway cars and a bus and those involved in the failed July 21 bombings against identical targets. Their investigation is focused on four suspects from the failed bombings whose images were captured by closed circuit television cameras and released Friday.

Mr. Blair appealed for help from Britain’s Muslim communities and said police were “still anxious for any sighting of the four individuals.”

In another possible connection, some of the July 21 attackers may have visited the same Welsh whitewater rafting center as two of the July 7 suicide bombers: Mohammad Sidique Khan and Shahzad Tanweer. The two bombers went rafting there on July 4, according to the National Whitewater Center.

Meanwhile, hundreds of relatives and friends of those killed in the July 7 explosions visited the sites of the attacks yesterday after they attended a briefing on the investigation. Many wept as they laid flowers at the bomb sites.