LONDON – Mercifully, there were no serious casualties and no fatalities. But the second wave of bomb attacks in London yesterday was intended to kill as many people as died on July 7 – only this time, just the detonators exploded.

It appears these were would-be suicide attackers operating in almost exactly the same way as before: three on the tube and one later on a bus, setting off their packages almost simultaneously yesterday and, symbolically, in the north, south, east, and west of the capital to form a “burning cross.” The fact that the explosions were small does not mean the devices were not murderous.

From witness accounts, and from what the Metropolitan Police Commissioner, Sir Ian Blair, said yesterday, it appears that the bombers attempted to trigger explosives concealed in backpacks.

Even though they struck at lunchtime, when fewer people travel on the Underground, the injuries would have been appalling had a bomb detonated on a train or platform. If it is confirmed that these were would-be suicide bombers, the grim conclusion to be drawn is that another four men, probably born and bred in Britain, were prepared to kill themselves to inflict terror on the capital. How many more are there?

Yesterday’s attack suggests that there is a network of cells, each containing four or five fanatics and possibly run by an Al Qaeda “specialist” who supplies backup and bomb-making expertise. In this case, however, the bomb-makers did not know what they were doing since none went off properly, indicating that this cell operated independently from the last.

Terrorist cells are structured so that each member is unknown to the others. This makes infiltration difficult and minimizes damage to the cell in the event of a member being captured. The fact that the police have recovered the explosives and may even capture the terrorists will give them some leads that would not have been available had the bombs gone off.

On the other hand, identifying this particular cell does not necessarily lead to others.

The identities of the July 7 bombers were rapidly established, yet this did not help intelligence agencies to track down yesterday’s attackers, or at least not quickly enough.

Again, the question that will be most exercising MI5 and the police is: How many more are there? These are not the first suspected cells to be identified. Last summer, two other alleged groups were broken up and arrests made. Their trials are due early next year.

Professor Paul Rogers, of Bradford University, said: “The one ominous thing is that this appears to be a group of a similar nature to the previous July 7 bombers. It implies there might be another cell primed and ready to attack.”

It is not unusual for bombs to fail to explode, though for all four not to go off suggests a level of amateurishness not present two weeks ago.

A 1-ton IRA bomb failed to explode in Canary Wharf in London in 1992 when the detonator went off without blowing the whole charge.

There was a similar escape for revelers in Birmingham’s clubland in 2001 when the detonator for a 70-pound Real IRA car bomb exploded but again did not trigger the main explosive.

Two years ago, two British suicide bombers traveled to Israel to carry out attacks there. Asif Hanif, from Hounslow, west London, killed three people, but his accomplice Omar Khan Sharif fled a nightclub when his explosive failed to detonate.

When the July 7 bombs went off, it was thought at first that the bombs had been left on the trains and bus and that a manhunt would be needed to find the perpetrators. However, they lay dead along with their victims.

Now, another huge trawl is already under way across the country to locate this particular group in the hope they can lead to others planning similar attacks.

It is a desperate race against time.