A fresh round of attacks in London has reminded the world of the depth of the threat from terrorism.
From Rome to Washington other major cities heightened their own security watches, while British leaders emphasized the need to remain steadfast in the face of what may be a terrorist campaign.
Follow-on strikes are meant to sow fear as much as shrapnel, say experts. They are enhancements to explosions which have already occurred - in this case, the deadly July 7 London transit bombings.
"We should not be surprised by this. ... The point is to make people think the previous target is still not safe," says Juliette Kayyem, a terrorism and national security expert at Harvard University's John F. Kennedy School of Government.
Although details of Thursday's explosions remained sketchy at press time, they caused little damage. The explosives either failed to go off, or were much less powerful than those of two weeks ago.
In that sense, this follow-on is reminiscent of shoe bomber Richard Reid's attempt to blow up a jetliner over the Atlantic in December 2001 according to Ms. Kayyem.
Mr. Reid may have been Al Qaeda's Team B, sent to reinforce the fear created by the Sept. 11 attacks. Yet his attempt failed, as his explosives failed to ignite and he was subdued by passengers.
"Team B is never as good as Team A," says Kayyem.
First impressions of the explosions on London's transport system on Thursday were that they were an attempt to replicate the July 7 attacks in conception, if not in casualties.
Three underground stations were evacuated shortly before 1 p.m. after witnesses reported lunchtime explosions on board trains, though it quickly emerged that the blasts were minor, amateur and far less destructive than the 7/7 attacks.
A bus was also quarantined in northeast London after another small explosion blew out windows. Television pictures showed it isolated in the Hackney district, with nearby buildings and homes also evacuated.
The Metropolitan police commissioner, Sir Ian Blair, told people to stay put and said the incident was "clearly very serious," as swaths of the subway network were temporarily shut down.
London's police chief said Thursday afternoon that there were still some unexploded devices on the city's transport system. He added that forensic evidence from Thursday's bombings could provide a "significant break" in the latest attacks. He added that it was not known yet if these attacks were connected to the 7/7 attacks.
After it became clear that the effect of the explosions would be minor, Prime Minister Tony Blair said that he hoped London would return to normal as soon as possible. While the attacks should not be minimized, he said, their intended effect should also be countered.
Such attacks "are done to scare people, to frighten them and make them anxious and worried," said Mr. Blair.
The similarities with 7/7 were unavoidable. Three affected Tube trains were again at three points of the compass - Shepherd's Bush in the west, Warren Street in the north, and Oval in the south.
Once again, streets and whole districts were taped off, the usual thrum of workers replaced by the urgent bustle of yellow-jacketed police and emergency services. Financial markets quivered, recovered.
Once again, witnesses described screaming and panic as Tube travelers helped each other to street-level safety. One witness at the Oval incident said a man ran away from a Tube carriage after his rucksack exploded. Another at Warren Street described a "popping" noise coming from the rucksack of a young man.
But the July 7 bombings killed 56 and left more than 700 injured. Thursday's follow-on, by contrast, caused no fatalities, and few injuries.
"The bombs appear to be smaller than on the last occasion," said police commissioner Blair.
Analysts said the explosions appeared to have been caused by detonators or dummy explosions.
"From what we can see this seems to be incredibly amateur," says Kirsten Parker, a senior analyst with Exclusive Analysis, a London-based strategic intelligence consultancy. "It seems that explosives weren't used, just detonators. There is a certain amount of copycatting involved."
The 7/7 attacks have been blamed on four young British Muslim suicide bombers, possibly with outside help from Islamic radicals affiliated to the loose network known as Al Qaeda.
Previous major attacks blamed on Al Qaeda radicals - 9/11, Bali, Casablanca, Madrid - have produced no follow-up attacks. The concern in London is that even if casualties from Thursday's incidents remain low, the threat of attack will hang over the city. "London is always going to be vulnerable," says Ms. Parker, adding that an already vigilant public would be even more alert.
Meanwhile, the effects of the bombing rippled around the world.
"We have a major [subway] facility that runs through the Pentagon," noted Defense Department spokesman Bryan Whitman.
In Bologna, a strong smell of gas prompted Italian fire officials trained to counter biological and chemical threats to sweep through a Dutch airliner parked on airport tarmac. No threat was discovered.In the US, where many transit systems are already on a high state of alert, security was increased at the Pentagon.
New York's mayor Michael Bloomberg announced Thursday that police will begin random searches of bags and packages passengers bring onto city subways.