The theory has been raised due to the large number of unused bombs found in a car the killers left at Luton railway station and at their "bomb factory" in a Leeds house.
When police raided the house after the attack, they found the bathtub filled with enough home-made explosives to make another 10kg bomb of the type used by Shehzad Tarweer, 22, Hasib Hussain, 18, Germaine Lindsay, 19, and Mohammed Sidique Khan, 30.
Lindsay's red Fiat was also found loaded with nine smaller bombs, leaving police speculating about the purpose of the excess explosives if the attackers intended to kill themselves.
The head of London's Metropolitan Police, Ian Blair, warned there was "a very strong possibility" of further attacks.
The toll from the attacks reached 55 on the weekend with the death of another victim in hospital, as a photo of the four bombers walking into a train station with their explosives in backpacks was released.
A closed-circuit television camera at Luton railway station, north of London, caught the bombers at 7.21am, just 90 minutes before the first of their devices exploded on Thursday, July 7.
Advertisement:
Police have also said the bombers could have been tricked by a "master" who told them they would have time to escape - when in fact the devices were set to go off immediately.
"We do not have hard evidence that the men were suicide bombers," a Scotland Yard spokesman said. "It is possible that they did not intend to die."
Lending weight to the theory is the fact that all four men had paid up their parking tickets before boarding a train at Luton for King's Cross, and that they all bought return tickets.
The speculation comes amid revelations that Khan, who blew himself up on a tube at Edgware Road, was scrutinised by Britain's MI5 last year, but judged not to be a threat to national security and not put under surveillance.
But in all this you got to wonder if they were ever planning to use the bombs, if the bombs that were used today were made to be faulty and if this is all a big scam to pysche Britain out into calling a war on terror...
do you just go out on the net and find this stuff, post it here, and make people think your smart? this is like, the 50th time i have seen someone do that, its so annoying...