Image of London bombers released

Splashed over most of Sunday's front pages, a security camera image released by police showed the four men, toting large rucksacks, at Luton station, north of London, about to head into the capital on July 7.

"Together, the four bombers," read the Mail on Sunday headline next to the grainy picture.

London's Metropolitan Police released the photo in the hope of jogging the memory of potential witnesses as part of an inquiry, which has now stretched to four continents.

Police had warned of possible further attacks, and on Saturday a British woman was among five people killed when a bomb destroyed a minibus in the Turkish seaside resort.

Turkish Prime Minister Recep Tayyip Erdogan condemned the blast at Kusadasi -- particularly popular with British tourists -- as a "terrorist attack", without blaming a specific group.

Police have now formally identified all four men suspected of carrying out the attacks on three London Underground subway trains and a bus which left at least 55 people dead including the bombers and injured more than 700.

They believe Mohammed Sidique Khan, 30, set off the explosion at Edgware Road station, while Germaine Lindsay, 19, caused the blast between King's Cross and Russell Square subway stations, the most deadly of the four rush-hour attacks.

The two other previously named suspects are Hasib Hussain, 18, and Shahzad Tanweer, 22.