London ? 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 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 British Transport Police. 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 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.

Posters of people who are still missing following Thursday's bomb attacks in London hang Sunday outside King's Cross Train Station in the British capital. Many people have taken to the streets of London, visiting sites to pay their respects to the dead and injured after the bomb attacks.

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.

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.