Investigators have yet to find evidence that last month's bombings against London's transit system were linked to each other or connected to an international terrorist network, Britain's top security official said Tuesday.

However, Home Secretary Charles Clarke (search) said he expects those connections to be discovered.

"I think it would be very, very surprising if they weren't linked in some way," Clarke said of the deadly July 7 attacks and the failed July 21 bombings in an interview with British Broadcasting Corp. (search) radio.

"There is not a direct linkage yet formally established."

The July 7 attack on three London subway trains and a bus by four homicide attackers killed 52 commuters.

Two weeks later, four people failed to fully detonate devices also placed on three trains and a bus. The men alleged to have carried out those attacks are in custody — three in Britain and one in Italy.

Clarke also said investigators were trying to determine if the attackers received support or training from outside groups, and he suggested they may have been part of a wider network.

"The full international links in relation to all this still remain to be fully clarified," he said.

Police have not charged anyone in the July 7 bombings.

Three of the alleged July 21 attackers have been charged in Britain with attempted murder.

The fourth, Hamdi Issac (search), is detained in Italy. Issac, 27, also known as Osman Hussain, was due to appear Wednesday in a Rome court, where Britain will press for his extradition, said Paolo Iorio, a lawyer representing the British government.

Clarke dismissed criticism that British security forces were aware that such attacks were being planned but failed to act.

"The fact is we did not know of these proposed attacks and that has been the very striking fact of what has taken place," he said.

Since the attacks, Britain has announced a range of anti-terrorism measures, including closing mosques linked with extremist clerics and deporting or barring radical Islamic preachers. Last week, British authorities detained 10 foreigners, including radical preacher Abu Qatada (search), with plans to deport them in the interests of national security.

Clarke can deport or exclude people from the country if they threaten public order or national security. He also is trying to broaden the criteria for taking such action.

After a two-week consultation period ends Friday, the government will have the power to act against anyone who foments, justifies or glorifies terrorism, fosters hatred or advocates violence through public speaking, preaching, running a Web site or publishing leaflets.

In a statement Monday, Clarke hinted that another group of foreign nationals could be taken into custody for possible deportation once the powers take effect.

"We are continuing to look at people in this country whose presence here is not conducive to the public good," he said. "We will be looking at further steps that can be taken to ensure that those who are working against the interests of this country are properly dealt with."

In northern England, police said they arrested four people under anti-terrorism legislation at Manchester (search) airport.

Two men, ages 48 and 23, and two women, ages 48 and 27, were arrested Monday for suspicion of providing money or property for terrorist purposes, police said.

Officers also searched a house in the central town of Dudley as part of the investigation, a police spokeswoman said.

The four suspects are not believed to be connected to the London bombings, the spokeswoman said.