Britons gathered in churches Sunday and piled bouquets of flowers at an Underground station to mourn victims of last week's bomb attacks on London's transport system as police sorted through hundreds of tips from the public.

Three men arrested at Heathrow airport (search) on Sunday under anti-terrorist laws were released later in the day without charge, police said.

Police had cautioned against linking the detentions of the men — all Britons — to the Thursday explosions on three subway trains and a double-decker bus in which at least 49 people were killed and 700 wounded. Sixty victims remained in hospitals Sunday.

Deep underground, police continued the hot, filthy work of searching for bodies from the worst of the subway bombings. Twenty-one bodies have been recovered so far in the tunnel between Russell Square and King's Cross stations, said Andy Trotter, assistant chief constable of the British Transport Police (search). Those victims are part of the total death count of 49.

Authorities have said they expected the death toll to increase.

In an interview with FOX News, Secretary of State Condoleezza Rice (search) said the attacks bore an "eerie familiarity" to the Madrid railway bombings that killed 191 people in March 2004.

"And so we're trying to help the British in any way we can," Rice said from Beijing.

Reports in London newspapers Sunday identified a possible suspect as Mustafa Setmarian Nasar — a Syrian suspected of being al-Qaida's operations chief in Europe and the alleged mastermind of last year's bombings in Madrid.

London police refused to comment, but Fran Townsend, President Bush's homeland security adviser, told "Fox News Sunday" that both nations were trying to locate Nasar.

As police studied 1,700 tips that have flooded in from the public so far, they also pored over surveillance camera recordings and appealed for more help from anyone with amateur video or images from camera-equipped cell phones taken near the four blasts.

"I would ask people across London to think very carefully about anyone they know whose behavior has changed suddenly," said Deputy Assistant Commissioner Peter Clarke, head of the Metropolitan 's Anti-Terrorist Branch. "Tell us what you see and what you know, and let us decide if the information you have is valuable or not."

A former London police chief, meanwhile, said the bombers were "almost certainly" British subjects, though investigators did not endorse the theory.

"I'm afraid there's a sufficient number of people in this country willing to be Islamic terrorists that they don't have to be drafted in from abroad," said John Stevens, who headed London's Metropolitan Police for five years until retiring in January.

Brian Paddick, deputy assistant commissioner of Metropolitan Police, cautioned that police had drawn no conclusions about the nationality of the attackers.

Stevens, writing in the News of the World, said "we have already convicted two British shoe bombers, Richard Reid and Saajid Badat, and there were the two British suicide bombers, Asif Hanif and Omar Sharif, who killed themselves in Israel."

Hanif was identified as the suicide bomber who killed three people and injured 60 on April 30, 2004, at a Tel Aviv nightspot, while Sharif allegedly fled from the scene and was later found dead.

Reid tried to detonate a bomb hidden in his shoe aboard a flight from Paris to Miami on Dec. 22, 2001. He is now serving a life sentence. Badat had bought a ticket for another flight, but changed his mind and took his bomb home to western England, keeping it in his bedroom.

It took police nearly two years to track Badat down. He pleaded guilty to a conspiracy charge in February and is serving a 13-year prison sentence.

Another major terrorism trial ended in April with the conviction of an Algerian, Kamel Bourgass, for conspiring to poison Londoners with ricin, a deadly toxin. Bourgass, who allegedly had ties to al-Qaida, was also convicted of murdering a police officer while resisting arrest.

Seven other Algerians and a Libyan charged in that plot were either found innocent or weren't prosecuted.

Claims of responsibility by two radical Islamic groups or simply the suspicion that Muslims were responsible apparently have spawned reprisals, police said.

Arson attacks on mosques in Leeds, Belvedere, Telford and Birkenhead caused little damage, the Association of Chief Police Officers said Sunday. There also have been scattered reports of verbal abuse and vandalism.

"We encourage everyone to report this type of obnoxious and dangerous behavior," association president Chris Fox said.

At St. Pancras Parish Church, 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 people came to a Garden of Peace hastily created at King's Cross Station, bearing flowers and cards, many intending simply to show solidarity.

"We are all Londoners, we are all united, even in grief," said Adebowale Badejo, 33, who brought his family to the garden.

Homeland Security Secretary Michael Chertoff said the United States would keep its terror alert on high for mass transit, while the U.S. monitors the British investigation for information about who was responsible and "what kinds of tactics we have to worry about."

Code orange indicates a high risk of attack, and in the U.S. system is the second-highest terror alert behind red. The lowest level is green, followed by blue and then yellow. Chertoff is considering changing the system because of complaints that it is too vague and confuses the public.

"I'd love to say we're going to see green in our lifetime," Chertoff told NBC's "Meet the Press." "It's kind of an aspirational state, but I can't tell you in the foreseeable future we're going to be below yellow."