Since 1972, when IRA bombings started on the British mainland, there have been many attacks aimed at high-profile targets, including Downing Street (1992), the London Stock Exchange (1990) and Canary Wharf (1997), one of London's tallest buildings housing mainly financial and media companies.

Londoners became accustomed to security scares, evacuations, and stringent searches, particularly around the political and financial centers. The IRA's bombing campaign, in pursuit of a British withdrawal from Northern Ireland, engendered a sprit of defiance — a "bunker" mentality — among many London residents who were determined not to let terrorist violence affect their lives.

Minor alterations were made to everyday life. Trash cans were removed from Underground stations, and a "ring of steel" — a series of checkpoints on all main roads entering the financial district — was installed to try to forestall attackers.

Some of these security restrictions, such as the checkpoint near Liverpool Street station, scene of one of Thursday's attacks, remain in place.

While the threat of IRA attacks has diminished in recent years following increasing political accord, Britain's role in the U.S.-led war on terror opened up a new fear: Islamic extremists.

In the four years since the attacks of Sept. 11, 2001, British authorities have been increasingly concerned that the country has been used as a training ground for terrorist activities.

On Dec. 22, 2001, Richard Reid, a Muslim fundamentalist from Gloucester, England, attempted to blow up an American Airlines flight carrying 190 people from Paris to Miami using explosives hidden in his shoes. Reid pleaded guilty at a court in October 2002 and is serving a life sentence in the United States.

Zacarias Moussaoui, who was charged with conspiracy relating to the Sept. 11 attacks and awaits sentencing in suburban Washington, also became radicalized while living in London.

More recently, Kamel Bourgass was convicted in April at London's Old Bailey criminal court for conspiracy to commit a public nuisance "by the use of poisons or explosives to cause disruption, fear or injury." Authorities told British newspapers they believed the failed asylum-seeker plotted to smear car door handles and contaminate toiletry containers with ricin or nicotine.

This resulted in a sense among the British public that a terrorist attack was inevitable.

In April, a Populus poll for The Times of London newspaper showed that 72 percent of the British population believed an al-Qaida attack was "inevitable," an attitude commentators described as "robust fatalism." But the same survey found only a tiny minority were altering their travel plans or normal routines because of the threat.

Also in April, London was coded as a "high risk" city in rankings prepared by Aon, the world's second-largest insurance broker, the same category as Baghdad, Kabul and Jerusalem. The map assigns ratings ranging from "severe" to "low" to each country.

At the time, Paul Bassett, an executive director of Aon, told the Times of London: "We're seeing increased recruiting of radical Islamists in Europe for a tour of duty in Iraq, then these people returning to their own country, feeling motivated and recruiting others. If they can carry out a successful attack around the election in the UK, they will."

Thursday's attack suggested the terrorists had a wide range of targets: one in the financial district, two in the heart of the tourist and university district, and one in an ethnically diverse area the west of the city.

The first explosion took place at 8:51 a.m. on a Circle Line train traveling between Liverpool Street, a major train hub connecting London with the east of the country, and Aldgate East, a station used by many who work in the financial center. In the financial district, this is close to two tourist sites, the Tower of London and Tower Bridge.

The explosion at 8:56 a.m. on the Piccadilly Underground line took place between Russell Square, home of the University of London, and King's Cross, another major train hub that connects the capital with the north of Britain including Leeds and Edinburgh, Scotland. Yards away, the bus bomb exploded in Tavistock Square at 9:47 a.m., in the heart of Bloomsbury, close to tourist destinations such as the British Museum.

Further west, there was a blast on the Circle Line a 9:17 a.m. near Edgware Road, an ethnically diverse area known for its Turkish cafes.

Sam Coates, a reporter for The Times of London, currently is a Laurence Stern fellow at The Washington Post.