By Jane Elliott
BBC News health reporter
As the UK comes to terms with the death of at least 50 victims in the London bombings, scientists and medics continue the grim task of identifying the dead.
Trying for positive identifications
So far 25 of those who died in last Thursday's bomb attacks in Central London, including three of the bombers, have been identified.
But, to avoid any distressing errors, the work has to be painstaking and slow so police have asked for families' patience while they analyse the evidence.
Many of the bodies are so badly injured they are impossible for relatives and friends to identify.
And in some cases, experts only have body-parts to work with.
This is where experts like forensic radiographer Mark Viner and his colleague Kim Hutchings can be vital.
They can use X-rays of the body to help create a positive identification of the dead person.
Kim Hutchings, based at Homerton University Hospital, London, helped identify victims of the tsunami disaster.
She said: "Radiographers are key part of the identification and autopsy process.
"It is possible to identify people by matching X-rays to the X-ray films they had done in life.
"The team would identify any previous distinguishing marks, pathology or injury, such as previous fractures, dental work or surgery, so that the pathologist could identify them."
Mark Viner has worked on many high profile cases
This might not be as obvious as a broken bone - for instance, old X-rays can show up unusual patterns of growth.
Medics can tell by looking at fractures whether they are new or old by the amount of healing that has gone on.
Even the tiny amount of healing that has gone in a fracture that is one or two days old can be enough for experts to distinguish between an old injury, or one sustained at the point of death.
The technique can even work to a degree for bodies that have no obvious distinguishing marks from their previous medical history.
Scientists can inch towards a positive identification by measuring limbs to discover their height, sex and sometimes even their age.
The fact that the bones do not fully fuse until the age of 25 can help pin down this last element.
Experts can call on high-tech X-ray equipment to help them.
Mark Viner, based at Barts and the Royal London Hospitals, helped in the identification of victims of the IRA bombings in Victoria Station and Canary Wharf, and worked on atrocities in Sierra Leone, Bosnia and Croatia.
He said that bodies can be scrutinised using a machine called a fluoroscope.
This speeds up the identification process by providing a "real-time" video shot, allowing fragments to be removed from the body while the X-ray is being carried out, rather than waiting until it is over.
In the case of the London bomb blasts, X-ray technology will not only help to identify victims - analysis of the pattern of injuries will provide vital clues about the nature of the explosions.
The work of identifying victims falls under the auspices of the Assocation of Forensic Radiographers.
A spokesman said it was important to remember that, unlike the tsunami, the London bombings were a crime scene.
As well as identifying the victims, medics also had an important role to play in preserving and identifying evidence that could be used in any subsequent prosecutions.