LONDON - Less than two weeks before they attacked London on July 7, three of the four suicide bombers traveled to the city for what appears to have been a practice run, the police said on Tuesday.
Police released closed-circuit TV footage showing the three suicide bombers met at Luton station at 8:10 a.m. on June 28 before traveling to King's Cross Station in London. The July 7 attacks killed 56 people, including the four bombers, and injured 700.
Scotland Yard also said the bombers made their explosives from peroxide and carried them in coolers - its first confirmation of ingredients revealed by New York City detectives who observed the London investigation.
British investigators said they found two unexploded bombs made from peroxide-based HMTP, and encased in nails in a car the attackers left at the Luton train station north of the capital.
They did not immediately specify how they knew the bombs that exploded aboard three London subways and a bus were made from peroxide-based explosives, which must be kept cool until used.
The bombs had been "effectively made" and that whoever made them had "done some good research and had been well trained," said Peter Clarke, head of London's Metropolitan Police anti-terrorist branch.
Andy Oppenheimer, an explosives expert with Jane's Information Group, said HMTP was one of a group of explosives made from common, easily available items such as hydrogen peroxide, or hair bleach.
He said peroxide-based bombs could be quite powerful, although not nearly as strong as plastic explosives.
Also, a suspect in London's July 21 bombing attempts told investigators the explosives he carried were made from a mixture of flour and a liquid hair product. Detectives located the images after finding train tickets and receipts indicating the timing of the trip.
The trip followed the same route the suspects are thought to have taken on the day of attack, police said.
Information from the New York Times and Associated Press was used in this report.