LONDON — The security scare lasted seven hours, closed one of Britain’s major intercity expressways, and drew in an armed counterterrorism unit, police helicopters, fire engines, ambulances, and scores of emergency workers. For a good part of Thursday, it was headline news on the country’s major broadcast networks.
And all because a man traveling on a bus to London tried to light up an electronic cigarette that smokers use as an aid in quitting, causing another passenger who spied vapor spiraling from the device to alert the police by cellphone.
By the time of the evening newscasts, the episode had taken on a somewhat comical edge, but behind it lay far more serious concerns that will be on the minds of millions around the world three weeks from now as thousands of athletes converge on London for the Olympics.
The risk of a terrorist attack on the Games has been a major concern for British officials — and for the nearly 200 nations and territories that will be competing — since the Olympics were awarded to London in 2005. The concern was amplified when Islamic militants bombed London’s transit system the day after the Games were awarded. The bombing killed 56 people, including the four bombers, in what was Britain’s worst terrorism attack.
Ten days ago, Jonathan Evans — the director general of MI5, Britain’s domestic security service — offered a heavily qualified statement of reassurance. He said officials with Britain’s terrorist-alert system had estimated the threat of a terrorist attack as ‘‘substantial,’’ the third-highest rating on a scale of five, meaning that a strike was ‘‘a strong possibility.’’
But in a rare public address, Evans said, in effect, that years of experience in monitoring terrorism threats and rounding up potential attackers since 7/7 — as the attacks of July 7, 2005, are known — and a much-improved system of international cooperation had vastly improved the chances of heading off a terrorist attack on the London Olympics.
‘‘There is no such thing as guaranteed security,’’ he said. ‘‘But I think we shall see a successful and memorable Games.’’
In briefings during the prelude to the Games, senior MI5 officials have elaborated on that theme. Their message has been that an international counterterrorist team involving agents from MI5 and its sister agency MI6, Britain’s secret intelligence service, and from the FBI, CIA, and other Western intelligence services, has built as effective a shield against an attack as is practically possible.
Still, events Thursday appeared to signal the fine balance of the preparations. The bus episode, which occurred near Birmingham, Britain’s second-largest city, and the arrests at dawn of five men and one woman in London for what the police have described as ‘‘suspicion of the commission, preparation, or instigation of acts of terrorism’’ suggest that Britain, for all its efforts, remains deeply apprehensive about the terrorist threat.
Scotland Yard said the London case was not linked to the Olympic Games, even though three of the suspects were arrested in a house less than a mile from the main Olympic stadium and the athletes’ village in East London. Eight homes and a business in various parts of London were being searched, according to a statement from Scotland Yard.
The statement said no attack was imminent, which appeared to align with what MI5 officials have said about a succession of terrorism-related arrests in Britain this year involving at least 23 suspects accused in a variety of plots.
Many of the suspects, like those arrested Thursday, are British citizens.
Officials say that the imminence of the Games has caused security agencies to move earlier to arrest suspects than they would have otherwise. This has meant, they said, that many suspected plots have been interrupted at an early stage, before any bombs have been built, weapons acquired, or targets selected.