As distraught friends and relatives continued to search frantically for 25 missing loved ones following the carnage in London, bomb disposal squad officers carried out four controlled explosions on a bus in central Birmingham and probed a box with wires coming out and a switch on top.

"The threat that we responded to yesterday was very specific," West Midlands police chief constable Paul Scott-Lee told a news conference.

"It was specific about the time and also the locations ... The people of Birmingham were in danger last night."

Deputy Chief Constable Stuart Hyde, meanwhile, discounted a direct link to the London bombing of three trains and a bus.

"I do not believe that the incident that we are dealing with this evening is connected with the events of July 7 in London."

The heart of Britain's second city in central England, 195 kilometres (120 miles) from London, was sealed off overnight but West Midlands Police told AFP the cordon had largely lifted by 6:00 am (0500 GMT).

As Birmingham gradually returned to normal, police continued their hunt for the culprits behind London massacre.

3 HELD AT HEATHROW
Police yesterday arrested three people at London's Heathrow Airport under anti-terrorism laws, but said no link had yet been established between the arrestees and the subway and bus bombings.

"Three people have been arrested under the Prevention of Terrorism Act at Heathrow Airport but it is not known at this stage whether those arrests are linked to this inquiry or not," a senior London police spokesman, Commander Brian Paddick, told a news conference.

"They are still in detention," he said and warned against linking the suspects to the attacks.

"I am told that it is inappropriate and pure speculation at this stage to be drawing any direct linkages with the attacks in London, and at this stage we are not in a position to give any further information," he told the press.

They were the first terrorism-related arrests to be announced since the deadliest peacetime attack in Britain on Thursday.

But "people are arrested under the Prevention of Terrorism Act very, very frequently. It happens on a weekly basis and it has happened again at this time," said Andy Trotter.

UK-BORN TERRORISTS?
Writing in the News of the World newspaper, London's former police chief, Sir John Stevens, said he believed the bombings were almost certainly carried out by Britons, not foreigners.

He said police had foiled at least eight similar attacks in Britain in the past five years.

Outlining his fears that UK based terrorists were behind the attacks, Lord Stevens said it was thought 3,000 British born or based individuals had been trained in al-Qaeda camps.

He said that authorities believed that either they, or others they had passed their knowledge on to, were most likely to be responsible.

The British police said the three subway bombs went off almost simultaneously, making it more likely they were detonated by timers, rather than suicide bombers. That means the bombers may still be at large and could strike again, they said.

RAT-INFESTED TUNNEL
Below King's Cross, workers were still trying to retrieve bodies from a hot, narrow and rat-infested tunnel.

Andy Trotter, deputy chief constable of the British Transport Police, said the search for bodies some 40 metres (100 feet) below ground in temperatures reaching 60 Celsius (140 Fahrenheit) was gruelling.

"It is extremely hot, very dusty and it is a great challenge for them to continue their work to recover the remaining bodies from the train underground," Trotter told a news conference.

An unknown number of bodies remained out of reach in crumpled remnants of a six-carriage train between the city's Russell Square and King's Cross stations.

SEARCH FOR LOVED ONES
Anxious relatives continued to scour London's hospitals in search of loved ones missing since Thursday's blasts -- the worst peacetime attacks on the British capital.

Walls, bus stops and telephone boxes close to King's Cross station, scene of the worst blast, were covered with photographs of missing people and appeals for information about them.

Well-wishers have left hundreds of bouquets of flowers outside the station, many accompanied by messages testifying to London's multi-ethnic and multinational mix.

"Christian, Muslim, Jewish, Hindu, Sikh, Buddhist. We are all Londoners," read one message scrawled on a Union Jack flag.

"This is not about religion, race, beliefs. This is about human beings killing human beings," read the simple message attached to a bunch of flowers in Russell Square.

Gous Ali, a Muslim whose partner Neetu Jain remained missing three days on from the attacks, condemned the perpetrators.

"It's awful sitting here as a Muslim," Ali told Channel Four television.

"I pray to God that God will punish them for what they have done," he added.