Police raised the death toll in London's terrorist bombings to 52 Monday as forensics experts identified the first of the victims — a 53-year-old mother of two from outside London. Prime Minister Tony Blair (search) promised a "vigorous and intense" manhunt for the attackers.

As workers searched the twisted wreckage for more bodies, millions of Londoners rode subways and buses to and from work, tense but intent on resuming their routines four days after the strikes.

"We won't let a small group of terrorists change the way we live," London's mayor, Ken Livingstone (search), said defiantly.

In a somber address to the House of Commons, his first since Thursday's attacks, Blair said it seemed probable that Islamic extremists were responsible for what he denounced as a "murderous carnage of the innocent."

No specific intelligence could have prevented the strikes, he said.

"Our country will not be defeated by such terror," he told lawmakers. "We will pursue those responsible wherever they are and will not rest until they are identified and ... brought to justice."

President Bush expressed solidarity with Britain on Monday, saying, "America will not retreat in the face of terrorists and murderers."

Officials raised the confirmed death toll, which had stood at 49, to 52 as workers searched for corpses in mangled subway cars marooned in a hot, dusty, rat-infested tunnel, and warned that the body count likely would climb.

"That will rise," Metropolitan Police commissioner Sir Ian Blair (search) said outside the King's Cross station near the site of the worst of the four bombings — an explosion that killed at least 21 people on one of the Underground's deepest lines.

"They still have to get underneath the carriages, and it is possible they will find more" bodies, he said.

Two other subway trains and a double-decker bus also were destroyed in the attacks, which wounded 700 people. Fifty-six remained hospitalized Monday, many in critical condition, officials said.

Police announced they had identified the first of the victims — Susan Levy, 53, of Hertfordshire outside London. Forensics experts have warned it could take days or weeks to put names to the bodies, many of which were blown apart and would have to be identified through dental records or DNA analysis.

"We are all devastated by our loss," said her husband, Harry, a London taxi driver.

London's University College initially said one of its cleaning service employees, whom it identified as Gladys Wundowa, 51, also was among the dead. But later, the college said Wundowa remained missing.

Public transit officials said the number of passengers using London's bus and subway network, which handles 3 million people on a typical day, was back to normal Monday.

But commuters were jittery, understandably rattled by numerous security alerts and evacuations triggered by travelers temporarily abandoning their bags, and some people played it safe by taking taxis. Among them was Ted Wright, chairman of the British Poultry Council, who took a cab to "hopefully put my wife's mind at rest."

British intelligence officials met over the weekend with their counterparts from the United States, Canada and about two dozen European countries to discuss possible leads, police said Monday.

London newspapers identified a possible suspect as Mustafa Setmarian Nasar (search) — a Syrian suspected of being Al Qaeda's operations chief in Europe and the alleged mastermind of last year's Madrid railway bombings. U.S. officials said both the United States and Britain were seeking Nasar.

Security officials in Poland, meanwhile, said Monday they searched the home of a British citizen of Pakistani origin in the eastern Polish city of Lublin in connection with the bombings. Poland's Internal Security Agency did not release the man's name and said he was not taken into custody.

A man with dual British-Moroccan nationality also mentioned as a possible suspect told The Guardian newspaper he had nothing to do with the blasts.

"Over 30 years I have lived in Britain, I have never been involved in violence or crime," said Mohamed al-Guerbouzi (search), who was convicted in absentia in Morocco in 2003 and sentenced to 20 years in prison in the Casablanca terrorist bombings.

With last week's attackers still at large, authorities warned the public anew of the danger of more strikes.

"We can't possibly assume that what happened on Thursday was the last of these events," said James Hart, the police official in charge of London's financial district. "There is absolutely no doubt that there are people out there who wish us harm, and we have to be vigilant."

Underscoring how tense London remained, police briefly closed several streets where most government offices are located — including Parliament, the Foreign Office, and 10 Downing St., where Blair lives and works — after a suspicious package was found, but it contained no explosives. Later, police evacuated the King's Cross subway station for a time and shut the Waterloo bridge over the River Thames; both were false alarms.

Britain's Islamic leaders, alarmed at reports of arson and other violence targeting mosques, called for calm in a statement urging people not to use the bombings "as a morbid opportunity to attack and undermine British Muslims" and asking Muslims to help find the terrorists.

The mayor and other officials laid flowers beneath a tree in Victoria Embankment Gardens, a park wedged between the Thames and London's busy theater district, and said they planned to erect a permanent memorial there.

As the families of people who haven't been heard from since the bombings lost hopes of finding them alive, passers-by continued placing bouquets, cards of sympathy, teddy bears and balloons outside the targeted Underground stations.

The messages mirrored the sadness and anger sweeping a city still coming to grips with the attacks.

"God bless London," said one placed on a mound of flowers outside the King's Cross station.

"May the perpetrators rot in hell," read another.