LONDON, England (CNN) -- London transport police are urging people to go to work as normal on Monday to show those behind last Thursday's bombings they have not cowed the British capital.
"We say that London is open for business. If we don't do that, then the terrorists will have won and that's not what we want," Deputy Chief Constable Andy Trotter of the British Transport Police said.
Police had "redoubled our efforts" to make people feel safe moving about the capital again, Trotter said.
In their search for clues to the attack that killed at least 49 people and left 700 wounded, police also are calling on the public to send in pictures and video taken with digital cameras or mobile phones at the bomb sites. (Full story)
Thursday's bombings on three underground trains and a bus in London came during a peak in commuter traffic.
Police also said three British men arrested Sunday at Heathrow airport under were released later in the day. The arrests were made under the nation's anti-terrorist laws, and authorities emphasized their arrests should not be linked to the bombings.
Meanwhile, authorities have warned Britain could face more terrorist attacks unless those responsible for the attack on London's transport network are captured.
Home Secretary (interior minister)Charles Clarke gave the warning on Sunday as investigators continued to try to identify the attackers, studying videotapes and photos as well as appealing for help from the public.
"Our fear is of course of more attacks, until we succeed in tracking down the gang which committed the atrocities on Thursday and that's why the No. 1 priority ... has to be the catching of the perpetrators," Clarke, Britain's Cabinet minister for law enforcement, told BBC television.
Throughout the country, authorities were on the lookout for any suspicious activity, implementing procedures in place for the heightened alert level.
Early Sunday, police lifted an evacuation order for central Birmingham, Britain's second largest city. The order was issued Saturday night, after police said intelligence indicated there was a threat to the area and asked hotels, bars, and clubs to screen patrons. ( Full story )
Once the evacuation was under way, police conducted extensive searches and tested one suspicious package at a hotel, finding it held no danger.
West Midlands police chief constable Paul Scott-Lee said early Sunday that intelligence had indicated the threat was genuine, though nothing dangerous was found. He did not describe the nature of the threat.
Authorities also worked to help people feel safe, keeping a strong police presence in virtually all busy areas.
Police also continued to comb through the Underground train bombed between King's Cross and Russell Square.
Trotter, Deputy Chief Constable for British Transport Police, said it was possible more bodies would be found.
"As you can imagine, the working conditions are extremely difficult. There's little natural air flow down there," he told reporters.
"There are five recovery teams working constantly throughout the day in very very hot, difficult, dangerous conditions."
The death toll remained steady at 49, Trotter said. But it was unclear how many people may be found in the back of the train or underneath it.
Brian Paddick, deputy commissioner of London's Metropolitan Police, said there were reports of 31 people missing.
Many people stood on the streets of London for the fourth straight day, handing out pictures of their loved ones in hopes they may be found alive.
There was no word of advancements in the investigation. Authorities said they do not know precisely how the bombings were carried out.
Police have said the string of bombs that ripped through three Underground trains went off within seconds and that the blasts were so strong none of the victims have been identified.
"All three bombs on the London Underground system actually exploded within seconds of each other, at 8:50 in the morning," Deputy Assistant Commissioner Brian Paddick said, revising original accounts that the train blasts occurred over a 26-minute span Thursday morning.
"It was bang, bang, bang, very close together," London Underground managing director Tim O'Toole said. Nearly an hour later, at 9:47 a.m., a fourth explosion tore through a double-decker bus. ( Timeline )
With more than 700 people injured in the rush-hour blasts, Scotland Yard Commissioner Ian Blair said he expected the total death toll to rise, but probably not above 100.
Blair said the attacks had "all the hallmarks of al Qaeda," and authorities have vowed to hunt down the bombers.
Blair's predecessor, former Metropolitan Police chief John Stevens, said Thursday's bombers were "almost certainly" British.
Stevens, who served as police commissioner for five years before retiring earlier this year, said there were about 200 people born and bred in Britain who were willing to attack.
"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, adding that police had thwarted eight attacks in the past five years.
Technical data and witness accounts suggest the bombs contained synchronized timing devices and were probably not triggered by suicide bombers, police said, adding that the bombs were composed of "high explosives" and probably not homemade material. ('Level of sophistication')
The bombs used in the attacks held less than 10 pounds of explosives each -- light enough to fit in a bag or knapsack, police said. (Bombs small, portable)
Police said they were still unclear whether the person who planted the bus bomb died in the explosion or was even onboard at the time. They said the device was in a bag and not strapped to an individual.Mourning and remembrance
Meanwhile Sunday huge crowds gathered outside London's Buckingham Palace for an ornate ceremony marking the 60th anniversary of the end of World War II.
David Prowse, a wing commander with the Royal Air Force, said there was a growing sense "of not being put off, not being deterred."
"We saw this on a daily basis during 1939 and 1945," he said, "and we're inspired by our veterans to carry on."
Queen Elizabeth II marked the anniversary by driving with her husband Prince Philip in an open-topped car. CNN's Richard Quest said this would boost the spirit of defiance exhibited by Londoners.
But amid Royal Air Force flyovers and celebratory music, people in London and throughout the country were also mourning victims of the worst attack the nation has seen since that war.
In her address at Sunday's celebration, the queen said: "It does not surprise me that during the present difficult days for London, people turn to the example set by that generation of resilience, humor, sustained courage, often under conditions of great deprivation.
"That example and those memories should be kept alive by younger generations as they, in turn, strive to keep the peace in our troubled world."
The Archbishop of Canterbury Rowan Williams gave an address at Westminster Abbey, and the queen placed flowers in commemoration of those who died in the war.
The queen and her eldest son, Prince Charles, have visited the wounded in London hospitals and sent a message of defiance to terrorists on behalf of the British people. ( Full story )
Londoners packed churches across the city to mourn the victims of the attacks and pray for peace and calm.
People streamed into St. Pancras Parish Church for a memorial service, near where the bus bomb killed 13 people.
"This will only make us more determined to live in peace and respect each other and we can all play our part in that," the UK's Press Association quoted the Rev. Paul Hawkins as telling the congregation.
"There are no Muslim terrorists. There are terrorists. There are criminals who claim to be Muslims. We all know that the vast majority of our Muslim friends and neighbors are as horrified and disgusted at what has happened as we are."
Memorial services also were held at St. Paul's Cathedral, where special prayers were to be offered and candles lit in honor of the victims, AP said.
And Britain's top religious leaders made a joint appearance Sunday, asking all faiths to unite in condemning and fighting terrorism
At the Vatican, Pope Benedict told the London bombers to stop their killings and prayed that God would change their hearts.
"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 Benedict 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."Agonizing search for missing
Scotland Yard said 49 bodies have been recovered and that more bodies were still trapped deep underground at the site of the deadliest attack. But the remains were so mangled and difficult to retrieve that authorities have yet to identify a single body.
Forensics experts were using fingerprints, dental records and DNA analysis to help put names to the bodies, Dickie said, adding it could be weeks before the identities are released.
Authorities said rescue crews were hampered by dangerous conditions -- including asbestos, rats and temperatures that reach 140 degrees Fahrenheit -- in the Piccadilly Line tunnel between the King's Cross and Russell Square stations, where at least 21 people died.
With no official list of the victims published, relatives and friends continued their agonizing search for missing relatives, as flowers, notes and appeals for information were piled outside King's Cross station.
Police opened a 24-hour reception center to provide information and support to families. Authorities said they have received more than 100,000 calls about possible missing persons.
Meanwhile, a government-prepared dossier says al Qaeda is targeting middle-class Britons to join its ranks, a newspaper reported Sunday.
According to The Sunday Times, the reports by the British Home Office and Foreign Office detailed how extremist recruiters were looking to Britain. The reports were drawn up in the aftermath of the March 2004 train bombings in Madrid, Spain. (Full story)
British Prime Minister Tony Blair said Saturday it was crucial to address terrorism's underlying causes, which he listed as deprivation, lack of democracy and the ongoing Middle East conflict. (Full story)
The attacks came as Blair was hosting the G8 summit in Scotland and a day after London was awarded the 2012 Olympics. (Bulldog spirit)
Blair's refusal to call off the G8 was a testament to his political and communication skills, CNN's Robin Oakley says in an analysis of Blair's handling of the attacks.
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