5 Suspects Held in British Bomb Attempts

Terrorism analysts said it appeared that the timing and location of the planned bombings were intended to coincide with Brown's first days in office. They also noted that Brown is Scottish.

"That was probably a major factor in this," said M.J. Gohel, chief executive of the Asia-Pacific Foundation, a London policy group that specializes in security issues. "Otherwise, it makes no sense as to why they would pick that particular moment as well as Glasgow as a target."

Gohel said the perpetrators may have been trying to fuel public pressure for a rapid withdrawal of British troops from Iraq, an issue that was already looming as a major test for Brown's government.

"This was a signal to the new government, a statement of intent and intimidation," he said.

Gohel noted that al-Qaeda affiliates were emboldened by the success of the March 11, 2004, train bombings in Madrid, which preceded general elections in Spain by three days. Voters subsequently chose a new government led by Prime Minister José Luis Rodríguez Zapatero, who had promised to end Spain's military involvement in Iraq.

Peter Clarke, Scotland Yard's chief of anti-terrorism operations, said the investigation was "fast-moving. It is no exaggeration at all to say that new information is coming to light hour by hour." Police were searching homes and buildings near Glasgow and in Staffordshire and Liverpool.

Clarke said "extremely valuable" forensic information was found in the two Mercedes sedans. He said the links between the Glasgow and London attempted bombings were becoming "ever clearer."

"I'm confident -- absolutely confident -- that in the coming days and weeks we will be able to gain a thorough understanding of the methods used by the terrorists, of the way in which they planned their attacks, and of the network to which they belong." He also said thousands of hours of closed-circuit television footage was being analyzed.

Scottish police, who encouraged the many witnesses at Glasgow Airport to hand over any cellphone and camera images of the incident, said they were receiving about 100 calls an hour from the public.

"We will not yield, we will not be intimidated, and we will not allow anyone to undermine our British way of life," Brown said. The country can expect "a long-term and sustained attack on values we represent" by extremists using different weapons, "whether planes or cars," he said.

British counterterrorism authorities had warned in April of a possible al-Qaeda plot timed to the recent change in government.

In a quarterly intelligence report leaked to the Sunday Times of London and published April 22, Britain's Joint Terrorism Analysis Center revealed that a senior al-Qaeda commander had advocated carrying out an attack in Britain before Blair left office. According to the report, intelligence officials had recovered a letter written by Abd al-Hadi al-Iraqi, a top al-Qaeda operative who was later captured by the CIA, detailing al-Qaeda's desire to attack Britain during "a change in the head of the company," apparently referring to the handover of power from Blair to Brown.

British intelligence officials have described al-Iraqi as the brains behind a failed plot in 2004 to bomb a London nightclub, power plants and a shopping mall. Five men were sentenced to life in prison in April for their role in that case, which was dubbed "Operation Crevice" by British police.

A witness during the trial testified that the ringleader of the plot met with al-Iraqi in Pakistan. The same witness, an al-Qaeda operative who is in U.S. custody, also testified that al-Iraqi met in Pakistan with two of the culprits responsible for the July 7, 2005, train and bus bombings in London, which killed 52 commuters in addition to the four attackers.

Special correspondent Karla Adam contributed to this report.