The plot for the July 7 transit bombings in London, which killed 56 people, may have been much larger than previously known, ABC News has learned.
Sources familiar with the investigation tell ABC News an additional 12 bombs and four improvised detonators were found in the trunk of a car believed to be rented by suicide bomber Shehzad Tanweer. Police believe the bombers drove the car to Luton, where they boarded trains to London.
"I believe that the explosives left in that car were left there for a second strike," said Bob Ayers, a London-based terrorism consultant with expertise in demolition. "But the Metropolitan Police responded so quickly, they were able to get to the car and take control of the car before the second team could get the explosives and leave."
ABC News obtained exclusive photographs, which show the devastation left inside the London subway lines after the July 7 attacks.
"There is considerable damage there," said Ayers, who analyzed the photographs. "You can see it's blown out the walls, it's blown out the sides, it's blown the roof. That was a good size bomb that that man took down there and set off."
ABC News also obtained photographs, which offer a first glimpse of the bombs used in the attacks.
The bombs were made of homemade high explosives. The materials used are widely available products, such as peroxide. Some were packaged like pancakes, and others contained nails for use as shrapnel. An X-ray image of one of the bombs found in the attacker's car trunk shows the deadly concoction.
"When you put the X-ray machine on it [the bomb], you see what is bulging on the sides of the bottle are nails -- many, many nails," said Ayers, while examining the photo. "And the nails are put there so that when the bomb goes off, the nails will tear tissue and kill people in the area. Bombs don't kill by concussion. Small bombs, they kill by the blast effects of fragments of glass or metal, and this is designed to kill people."
British authorities are deeply concerned they are in a race against time against people who want to plan another attack.
ABC News' Pierre Thomas filed this report for "World News Tonight."