The Hardest Count

Benedetta Ciaccia, 30, an Italian-born business analyst, was among thousands of commuters on the way to work on July 7 as suicide bombers blew themselves up on three London Underground trains and a bus. Her friends and relatives have not heard from her since, and she was not among the 47 victims who had been positively identified nine days later. "We're still waiting," Fiaz Bhatti, Ciaccia's fiancé, told TIME last week. During the wait, scores of police, medical and forensic experts were engaged in the grim but necessary task of trying to establish the identity of the victims — which is why Bhatti has been telling police about the birthmark under Ciaccia's left eye.

Friends and relatives are spreading such details in hopes that some aspect will help identify the missing: Karolina Gluck, a 29-year-old administrative worker from Poland, has a pierced belly button and carries a London 2012 Olympics key ring; health-care analyst James Mayes, 28, who was identified late last week, had hazel eyes and short brown curly hair.

As recovery crews and police investigators continue their work, forensic experts in surgical scrub suits are trying to identify the bodies of the dead at a makeshift mortuary at a military barracks in east London. The work inside the white marquees has been slow, painstaking and complex, leading to criticism from families of some of the missing. Graham Russell, whose 28-year-old son Philip died on the bus, spoke for many when he told reporters: "Any delay is crucifying people."

The experts are sympathetic, but argue that the identification process has been complicated by several factors. One of them is sheer size: in a typical year, London has only about 200 murders, so this attack represents a homicide wave. Second, bodies were severely damaged by massive explosive forces. And finally, coroners are desperate to get things right. After 58 tourists were gunned down in a 1997 attack in Luxor, Egypt, by contrast, some bodies were misidentified and sent to the wrong countries. Andrew Reid, one of two London coroners overseeing the identification process, has warned that it might take weeks for all of the bodies to be recovered and identified. "We understand the distress," Reid said, while stressing the importance of returning "the right victims to the right families."

Though London has no experience with suicide bombers, the recovery and identification efforts are drawing on a growing body of international expertise. Police chiefs in Israel, where more than 500 people have died in suicide bombings in the past five years, describe Britain's Forensic Science Service as the world's best. Scotland Yard's officers have also been gathering experience. After last year's Indian Ocean tsunami, the Yard sent officers to Thailand to help match information about British victims. Others traveled to New York City to observe mass casualty identification procedures after the Sept. 11, 2001, attacks.

There is a protocol to such tasks. Standard international procedures begin by designating the mortuary site and forming an Identification Commission. In the London case, the commission is headed by Reid and the Westminster coroner, Paul Knapman, who led the inquest into a 1996 IRA bus bombing that killed the perpetrator; it also includes Metropolitan Police Service officers, forensic pathologists and dental experts, who act much like a tribunal in ruling on identity. Commission members study information provided by friends, relatives, doctors and dentists of the missing, and compare it to their own postmortem findings from victims' bodies, clothing and personal effects. All bodies are photographed, X-rayed and visually examined; some undergo a full autopsy.

The most reliable clues to a dead person's identity are known as primary criteria — fingerprints, dental records, DNA or implanted medical devices marked with a serial number. DNA matching is the gold standard; it requires comparison between DNA from human remains and that found, say, on a hairbrush or toothbrush belonging to the missing person, or with a close relative's DNA. The family of a woman still missing last week, 27-year-old Rachelle Lieng Siong Chung For Yuen from Mauritius, provided police with her toothbrush. Chris Hadkiss, a manager with Britain's Forensic Science Service, says his laboratory has so far analyzed DNA samples only from the suspected bombers. But he says it's "inevitable" that the lab will get around to studying DNA from victims whose bodies prove difficult to identify by other means.

Dental records are highly reliable; in many of the inquests that opened last week, they were cited as the basis for identification. The same was true after last year's tsunami. Pornthip Rojanasunan, a Thai forensic scientist who named 2,400 of the roughly 6,000 who died in Thailand, says, "The most useful method in identifying [tsunami] victims was their dental records." Coroners also rely on possessions — clothing, footwear, jewelry, watches, eyeglasses, together with scars, moles, birthmarks, tattoos and identity papers the person may have been carrying.

But for all the potential evidence available, Israeli experts expect that identifying the London victims will be very difficult. "Everything will be new to them," says Azi Zadok, head of the Israeli police's Forensic Science Division. "An exploded bus can be full of melted and burned bodies in large numbers with body parts scattered everywhere ... The bodies can be completely dismembered, with many parts scattered over hundreds of meters."

Even without such gruesome details, for those still missing relatives or friends, every passing hour is agony. "We have not heard anything," says Billy Chung For Yuen, Rachelle's husband. The uncertainty is only prolonging his suffering.