British police announced some progress Sunday in the painstaking effort to recover bodies still trapped underground after last week's subway bombings, while authorities expressed concern Britain could face more terror attacks if the perpetrators aren't caught.

Deputy Assistant Commissioner Brian Paddick of the Metropolitan Police also said three people were arrested Sunday at Heathrow Airport under the country's anti-terrorism laws, but he refused to link the suspects to Thursday's explosions.

Londoners, meanwhile, packed churches across the capital to mourn the bombing victims, begin healing and pray for calm as Britain's top religious leaders cautioned against retaliating against Muslims for any perceived involvement in the attacks.

The confirmed death toll from the subway and bus bombings was 49, but there has been confusion about how many bodies remained in the train hit deep underground between King's Cross (search) and Russell Square (search).

Andy Trotter, assistant chief constable of British Transport Police, told reporters Sunday that the figure of 49 included bodies counted in that wreckage, but "there is a possibility as the search teams go through that we will find more."

Ian Blair, commissioner of Metropolitan Police, said last week that the death toll was "50-plus."

Paddick said Sunday that an unspecified number of bodies had been recovered, but conditions underground were still "very hot, very dusty, very dangerous.

Difficult conditions — including temperatures that spike to 140 degrees, asbestos and rats — have hampered the recovery effort.

The deputy police commissioner also issued an appeal for videos, photos and cell phone images that might provide clues to the attackers' identities, setting up a special e-mail address.

If people weren't able to e-mail the pictures, they could call police to make other arrangements, Paddick said, adding that some 1,700 calls had been made to a special anti-terrorism hotline seeking information.

Investigators have been poring over videotapes from surveillance cameras and photos and appealing for help from the public.

Home Secretary Charles Clarke, the Cabinet minister responsible for law and order, told British Broadcasting Corp. TV that terror always has been a threat and warned Britain could face more attacks if those responsible for the bombings aren't found.

"The fact is the terrorist threat is a real one as we saw so dramatically and awfully on Thursday," he said.

"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 Number One priority ... has to be the catching of the perpetrators," he said.

Paddick told reporters it would be "inappropriate and pure speculation" to connect Sunday's arrests at Heathrow to the subway and bus bombings, saying the detentions were "routine."

"Three people have been arrested under the Prevention of Terrorism Act at Heathrow Airport," he said. "Those people are still detained."

The city's shock — with many questions still unanswered — was evident Sunday as people gathered for memorial services across the capital to remember those killed and the 700 wounded.

At St. Pancras Parish Church (search), just steps from where one of the bombs cut apart a double-decker bus and killed 13 people, the Rev. Paul Hawkins spoke of the diversity of culture and faith in London.

"This will only make us more determined to live in peace and respect each other and we can all play our part in that," he said.

Hundreds of fliers, photos and calls for help appeared pleading for information about loved ones not heard from for days and scores of flower bouquets were placed outside King's Cross subway station.

With the whole country on heightened alert, police on Saturday night evacuated 20,000 people from the entertainment district in central Birmingham after intelligence suggested a security threat.

Crowds in the city center faced "a real and very credible threat," police Chief Constable Paul Scott said Sunday. He declined to describe the nature of the threat, but said the danger had passed.

Police carried out one controlled explosion on a suspect bag on a bus in Birmingham, but concluded it was not a threat. Scott said the suspect package was not related to the larger security threat.

Birmingham, 110 miles northwest of London, was the target of one of the worst Irish Republican Army bombings of the 1970s. Twenty-one people died when the IRA bombed two pubs on Nov. 21, 1974.

Investigators, who on Saturday revised the timings of the subway blasts to near simultaneous instead of almost 30 minutes apart, are looking at claims by two al-Qaida-inspired groups that they carried out the bombings.

Former Metropolitan Police chief John Stevens said the bombers "almost certainly" were homegrown and that Islamic groups were actively recruiting and training as many as 200 British-born extremists to continue the attacks.

"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. He said police had thwarted eight attacks in the past five years.

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

Europol spokesman Rainer Wenning declined to comment on the reports.

However, al-Jazeera, the Arabic news organization, reported on its Web site that it had interviewed al-Guerbouzi, quoting him as saying British authorities knew his address in London.

Al-Guerbouzi was convicted in absentia by Morocco in December 2003 in connection with the Casablanca terrorist attacks and sentenced to 20 years in prison. Britain never extradited him.

Mustafa Setmarian Nasar — a Syrian suspected of being al-Qaida's operations chief in Europe and the alleged mastermind of last year's Madrid railway bombings — has emerged as a suspect in the London attacks, according to unidentified investigators cited in The Sunday Times, The Sunday Telegraph and The Mail on Sunday.

London's Metropolitan Police said they would not comment on the names of any suspects.

Police have concluded that the blasts happened within a minute of each other around 8:50 a.m. Thursday, suggesting the explosives were detonated by timers rather than suicide bombers. The bus was hit nearly an hour later.

Initial investigations showed the bombs contained high explosives, suggesting the material was not homemade. It was possible the explosives were industrial or military materials bought on the black market, police said, although investigators said it was too early to pinpoint the explosives' origins.