Mr Shakil and Mr Saleem were from Beeston in West Yorkshire, where Mr Saleem lived most of his life. He had recently moved to Tower Hamlets in east London.
One suspect had been arrested at an address in Leeds two weeks ago while two others were arrested at Manchester City airport as they "attempted to leave the country".
Deputy Assistant Commissioner Peter Clark, head of the Metropolitan police's anti-terrorism unit, told reporters at a news conference this lunchtime that today's development was the culmination of 21 months of "intensive investigation".
"To some it will bring back awful memories of that terrible day. For others there may be some relief," he said.
The investigation has so far received 15,000 statements and followed over 19,000 leads, while 966 people have sat in interviews, DAC Clark revealed.
He described the detailed evidence as "a complicated jigsaw with thousands of pieces".
"We now have enough of the pieces in the right place so that we can see the picture which is far from complete. Because of that, the search is not over. I firmly believe there are other people who have knowledge of what lay behind the attacks… knowledge that they have not shared with us. I know it for a fact," DAC Clark said.
He appealed for witnesses, especially in the West Yorkshire area, to come forward and suggested that many of those with relevant information had been actively deterred from approaching police.
"Surely this must stop. The victims of attacks, and those who will become victims of terrorism in the future, deserve your support," he added.
In addition to the 52 fatalities over 770 people were injured on July 7th. At 08:50 BST three bombs exploded on the Underground tube network, with a fourth bomb blowing the roof off a number 30 bus in Tavistock Square.
Since then the attack's perpetrators, Hasib Hussain, Mohammad Sidique Khan, Germaine Lindsay and Shehzad Tanweer, have been identified, but a report published last May investigating their motives failed to establish clear reasons for the attacks.
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