By Ioan Grillo

MONTERREY (Reuters) - Even if someone had not chopped the heads, hands and feet off the victims first, the chances of investigators identifying the 49 bodies dumped on a road in northern Mexico this weekend would have been far from certain.

Catching the killers may prove an even harder task.

More than 8,000 corpses left across Mexico since 2006 have not been identified, according to the National Human Rights Commission, a government-run body.

At the same time, more than 5,000 people who have disappeared during the drug violence sweeping the country have never been tracked down, according to the commission.

The failure to find and identify victims of massacres and murders has spurred criticism of the war on drug gangs waged by President Felipe Calderon since he took office in late 2006.

The thousands of grieving families searching for lost loved-ones are also likely to haunt Calderon's successor, who will be chosen in presidential elections on July 1.

"When you can't find your family member it completely destroys you," said Irma Hidalgo, whose 18-year old son was dragged from her home in Monterrey in 2011. "We just want to know if he is alive - or if not, then lay him to rest."

Hidalgo immediately called the Monterrey morgue following the discovery of the pile of dead outside the city on Sunday, but police told her not to come in because they said the bodies were so badly mutilated they were only identifiable with DNA.

She has already given her DNA samples to state and federal police and is assured they will be compared with the victims.

FORENSICS NIGHTMARE

The sheer scale of the carnage makes it difficult for authorities to handle the murder cases. The latest violence has pushed the total number of drug-related deaths to around 55,000 since Calderon assumed power in December 2006.

Emergency teams have had to confront mass graves with more than 200 corpses, dozens of bodies thrown into mine shafts and massacres such as the 49 headless corpses left on a road about 18 miles east of this industrial city.

State-employed forensic scientists complain it is an almost insurmountable task.

"I used to just deal with traffic deaths and the odd crime of passion. I never thought I would see this kind of devastation in my country," said a forensic scientist in Monterrey, who asked his name not be used. "I had a day recently when I had to look at five bodies that had been shot in one street, then three in another and then five corpses that had been cut up."

"At first you feel sick when you have to take evidence from bodies that have been decapitated and mutilated. But then you get used it and you don't feel anything," he said.

It has also been a challenge to store so many corpses.

Following the killings on Sunday, forensics vehicles needed to make several trips to take all of the 49 cadavers back to a morgue located in a public hospital in Monterrey.

Hospital officials said they just managed to fit the corpses in their fridges. In other cases, such as when mass graves were found by the nearby town of San Fernando in 2011, the government had to send refrigerated trucks to store all the victims.

CRIMINALS FOR COFFEE

Calderon defends his offensive by saying his government has made record seizures of marijuana, cocaine and methamphetamine, and arrested or shot dead dozens of major traffickers.

"There are people who criticize my government for fighting criminals, but what do they want? To invite (criminals) for a cup of coffee?" Calderon asked as he inaugurated a hospital in Mexico State recently. "If someone doesn't want to fight criminals, they shouldn't be in government."

But critics point out that even as soldiers and police round up drug smugglers, they fail to solve most murders.

Fewer than 20 percent of homicides in the last two years have led to arrests and convictions, according to police and court statistics. With such a backlog, cases of disappearances are given even less priority, family members complain.

Jorge Varestegui has been searching for almost two years for his brother and nephew, who were taken by armed men at a road block near his town of Parras in northeast Mexico.

"Police don't take the cases of disappearances seriously. They say the person probably ran away or something, even when you bring evidence to show they were kidnapped," he said.

With so many crimes unsolved, police cannot identify if many of the victims were actually criminals killed over the fight to smuggle drugs or simply civilians caught up in the bloodshed.

"The government gives a simplistic explanation that this is just criminals killing criminals. But how do they know, if they don't solve the crimes?" asked Indira Kempiris, a human rights activist in Monterrey. "Many of the victims are from the most vulnerable sections of society with no means to seek justice."

UNKNOWN MIGRANTS

A day after the 49 bodies were dumped, police said they had not yet identified any victims nor had reports of a mass disappearance lately, signaling the victims may have been foreign migrants passing through Mexico to the United States.

Graffiti at the crime scene bore the letter "Z", a symbol for the Zetas drug cartel, known to dominate the area.

As well as from drugs, the Zetas make money from human trafficking and extorting migrants, and have murdered many who fail to pay them. In 2009, the Zetas were blamed for a massacre of 72 foreign migrants, again in San Fernando.

With many families of victims living in poor communities in Central and South America, it is even harder to identify them. Many of them are undocumented migrants, further complicating their identification.

The continued failure to put names on so many victims leads to a vicious circle, in which gangs are encouraged to think that they can get away with murder, said Victor Clark-Alfaro, director of the Bi-national Center for Human Rights in Tijuana.

"Over the years, the state has accumulated a huge sack of dead people on its back without resolving the cases or punishing those responsible for them," he said.

(Additional reporting by Dave Graham; Editing by Anthony Boadle)