LONDON- Were the bombers in the latest attack on London copycats trying to increase fear - or was the al-Qaida terror network trying to send a message that they can attack at will?
The small explosions that again sent shudders of terror through the city Thursday - exactly two weeks after suicide bombers killed 52 - caused widespread disruption and dashed hopes that the first deadly bombings were a one-time event, as they so far have been in the United States and Spain.
In trying to determine the source behind the latest bombings, analysts considered the style and relative lack of success of the attacks.
"These attacks don't look like they were a hallmark of any one group," Paul Beaver, an independent security and defense expert, told The Associated Press. "They don't fit into any clear patterns."
Two devices detonated - one at the Oval subway station, the other on the No. 26 bus - but the other two apparently did not, Beaver said, citing security officials close to the investigation.
"It all seems a bit amateurish," he said.
But the fact that the latest strikes caused much less damage than the previous blasts didn't necessarily indicate a different group was at work, said Charles Shoebridge, a security analyst and former counterterrorism intelligence officer with the Metropolitan Police.
"If the same batch of explosives was used in both incidents, it's possible - as sometimes happens with homemade explosives - that the material had deteriorated over the two-week period," he said.
Investigators looking for similarities between Thursday's explosions and the ones two weeks ago were probing the attackers' patterns and the type of explosives used.
While two groups associated with the al-Qaida terror network quickly claimed responsibility for the July 7 bombings, there was no claim of responsibility for Thursday's attacks.
In the first set of attacks, all four bombers died. On Thursday, two men were arrested in connection with the attacks, while it appeared at least two other attackers were still at large.
Although officials say they don't yet know what type of explosives were used in the July 7 bombings, some reports have speculated attackers used the explosive TATP, or triacetone triperoxide, which can be made from commercially available products but degrades rapidly.
Investigators were checking whether TATP was used in Thursday's attacks. They should be able to nail that down because "significant amounts" of material were left behind, said detectives at the scene.
Whether investigators are able to pinpoint the groups responsible for both attacks is not as important as getting a handle on the numbers of splinter terror cells operating in Britain, analysts say.
Intelligence experts have admitted that they've been unable to penetrate radical elements in Britain so far, and even though they are working to create an undercover squad, it will take time to understand how deeply rooted the groups are.
Though Thursday's attacks did little damage, they did put people on edge. In a city where more than 3 million residents rely on public transport, commuters were clearly rattled, and the government faced increased pressure to answer questions about safety.
The latest attacks proved an embarrassment for police and underscored the difficulties authorities face in blocking other attacks. The attackers were able to detonate the explosions while Britain was still under alert and investigators were tracking down leads from the previous bombings.
No immediate security precautions had been put into place following the bombings. Although sniffer dogs were being used, officials were carrying out only spot checks of backpacks and purses. No precautions were given to commuters ahead of Friday's morning rush hour.
One of authorities' greatest fears is that an audacious attack will inspire similar attacks, said Rachel Bronson, director of Mideast Studies at the New York-based Council on Foreign Relations.
"It's all done to sow terror, and there's nothing more terrifying than bombs followed by bombs," she said.
Vince Cannistraro, a former counterterrorism chief at CIA, said there are still any number of possibilities as to who was behind the latest attacks.
"These were either unsuccessful suicide bombers, or they were just detonators to try to scare the hell out of people - unless they had terribly bad luck and the bomb material all fizzled out," he said.
The latest attacks, though, were "not the attempt of four professionals to set off a bomb," he said.
Associated Press Writers Lee Keath in Cairo and Katherine Shrader in Washington contributed to this report.