British police are appealing to the public for any relevant photos, video footage or mobile phone text messages from Thursday, the day of the bomb attacks on the London transit system.

Police are also taking the unusual step of asking Internet and cellphone companies to send along any data that might be relevant.

Bombs that killed 191 people in the Madrid attacks in March 2004 were detonated by cellphones programmed to sound alarms simultaneously.

Hours after the Madrid attacks, police discovered a gym bag containing an unexploded bomb and a cellphone. Tracking the phone's earlier activity helped lead to arrests.

British police are now looking for similar information, and are hoping that mobile phone companies will retrieve data from their systems to display any suspicious calls their users made from the area of the attacks Thursday morning.

Given the highly coordinated nature of the attacks, it's likely the bombers used cellphones to communicate, as well as to trigger to the explosions.

Rounding up suspects

Meanwhile, British police said they arrested three people Sunday at London's Heathrow Airport under the country's anti-terrorism act, but would not link the suspects to last week's terror attacks. The men were released later in the day.

Although police were careful not to publicly speculate, some authorities warned that they feared more attacks.

"The fact is the terrorist threat is a real one as we saw so dramatically and awfully on Thursday," Secretary Charles Clarke, the Cabinet minister responsible for law and order, told BBC television.

The focus of the investigation remains in Britain. Former Metropolitan Police chief John Stevens said he believed the bombers were "almost certainly" British residents.

"They are also willing to kill without mercy -- and to take a long time in their planning," Stevens wrote in an article in the News of the World newspaper Sunday.

British media said investigators asked their European counterparts, including Europol, to search for Mohamed al-Guerbouzi, a 44-year-old Moroccan who was once given asylum in Britain.

However, al-Jazeera, an Arabic news organization, reported that it interviewed al-Guerbouzi. He was pictured with his head covered, saying British authorities knew his address in London.

Al-Guerbouzi was convicted in absentia by Morocco in December 2003 for a terrorist attack in Casablanca and sentenced to 20 years in prison. He was never extradited.

According to unidentified investigators cited in several newspapers, Mustafa Setmarian Nasar has also emerged as a suspect.

Nasar is Syrian suspected of being al Qaeda's operations chief in Europe. He is widely believed to be the mastermind of last year's Madrid bombings.

Church leaders address attacks

The confirmed death toll from the London attacks is at 49, but police believe it will inevitably rise above 50 after search crews remove bodies that are still trapped in the subway system.

Forensic experts are relying on fingerprints, dental records and DNA analysis to identify the victims. None of the 49 dead have been formally identified yet.

As search crews continued their recovery efforts, Londoners congregated at memorial services to mourn the dead and missing victims.

In a service to mark the 60th anniversary of the Second World War, the Archbishop of Canterbury, leader of the Church of England, likened the horror of the war to Thursday's attacks.

"Today of all days, we need to reminder that the spirit of murder and humiliation is still abroad," he told worshippers in Westminster Abbey.

"There is a generation of people for whom the sight of a devastated, bombed London will bring back harsh memories."

At the Vatican, Pope Benedict prayed that God would change the hearts of the perpetrators.

"We pray for the people killed, for those injured and for their loved ones. But we even pray for the attackers: Lord, touch their hearts," Reuters quoted the pope as telling the crowds in St. Peter's Square.

"To those who foment feelings of hate and carry out such revolting terrorist acts, I say: God loves life, which he created, not death. And I say, stop, in the name of God."