The first time I covered the drug war in Monterrey was in 2007, when the uptick in the violence was just beginning to be felt. I travelled there as a producer and we recorded a chilling story about how the war between rival cartels was effecting everyday Mexicans. A little more than 200 people were killed that year - including politicians and high-level police officers.
But nothing prepared me for my week-long reporting trip in August 2012. At least three people were killed each day we spent in the city. The murders were gruesome, involving teenagers caught in the crossfire, drug dealers shot down by police and victims hanged from bridges in broad daylight. The number of murders in the past year alone exceeded 1,600.
The upsurge in violence reflected a national trend over the past six years - since President Felipe Calderon declared war on the drug cartels. Monterrey is the second largest city in Mexico, and an industrial powerhouse. Home to dozens of major companies and factories, the city contributes more than 7.5 per cent of the gross national product. People here have a saying: "If Monterrey goes, then so does the entire country."
Three major cartels are all at war in this city of more than four million people. The Sinaloa Cartel (allegedly controlled by Joaquin "El Chapo" Guzman), the Gulf Cartel and the infamous Zetas Cartel (reportedly started by former members of the military) are all killing each other over control of the lucrative drug routes that run through this city. The top prize is getting drugs across the border to the United States, the billion-dollar market as it is understood to be home to the largest number of drug consumers in the world.
One of the most moving stories we have reported was about the thousands of dead bodies that turn up, or are discovered, but never claimed. More than 16,000 corpses throughout the country remain nameless. Some of them are so mutilated that only fragments remain.
The problem with identifying bodies in Mexico is enormous and complex. For starters, not every state is equipped with a forensics lab. President Felipe Calderon began the process for a national DNA bank but it has yet to be created. Often, bodies of people who disappear in one state show up in another - but it can take months for the DNA samples to be matched, because the information is not readily available between states.
Another complication is that people are often afraid to give DNA samples, because they don't want to admit that their loved ones could be dead. Trusting investigators and government authorities is also a tough task for many Mexicans, because of widespread corruption. Often, the same people working in public security - such as police officers - might also be working for criminal gangs. Distrust is a significant reason why people are often reluctant to give sensitive information, such as DNA, to the authorities.
But reuniting families with their loved ones - even if they are deceased - is still an important process, and does provide relief for some who want answers about those friends and relatives who have disappeared.
One of the most powerful stories we heard while filming was from Dr Lourdes Chavez. She decided to become a forensic expert because she wanted to perform clinical work instead of dealing directly with patients. She confessed to me that, while she was in medical school, she always felt it was too overwhelming to have to speak with families of ill patients - so she preferred to stay behind the scenes. But the explosion of homicides in her city has meant that she is now more like a detective trying to solve gruesome murder mysteries than "just a scientist".
The number of DNA samples she and her team have to analyse has increased four-fold since 2010.
"The most difficult thing is when you have to tell a family that all that is left of their loved one is a small fragment," she tells me about her job. "That's it. No school ever prepares you for this."
But the most overwhelming moment came when we met the Pe?a family. Gerardo and his wife Alma are typical working class Mexicans. They both work hard and struggled to raise their two children - Gerardo and Karen - only one year apart in age. Gerardo Junior, a shy but popular boy, loved the camera his mother bought him for Christmas and wanted to become a photographer. After he graduated high school he went to work with his father at the factory. One Sunday night in January 2011 he told his parents that he was going to a party with friends from the neighborhood. He left the house - but never came home.
That very night his father got a chilling call from his son. He picked up the phone and heard Gerardo say in a calm voice: "Dad it's me. The Zetas have me." His father almost didn't believe him until he heard a man grab the phone and say: "If you don't give us $4,000 we will cut your son into little pieces."
Gerardo and his wife Alma quickly ran to find the parents of the other boys who had gone to the party with their son. All of them got similar calls.
Relying on friends and family, they collected the money and the very next day they dropped the cash off in a black bag where the kidnappers told them. Then they waited for a call or some proof of life but it never came.
After nearly a week they finally decided to go to the police. "We were scared to file a report," said Alma. "We thought that if we went to the police, the gang would kill our son."
Months went by, and with no sign of his son, Gerardo decided to look himself. He showed photos of the 19-year-old to everyone in the neighbourhood and even went to see gang members begging for information.
"One day people in my family came to me," Gerardo tells me as he moves nervously in his chair on the front porch. "They told me to stop talking to the drug gangs because I was going to create problems for everyone." He says that's when he realised he had to take a step back.
Finally, in April, he and his wife decided to give samples of their DNA to the police.
"We didn't even know we had to give samples," says Alma. Like many families searching for the disappeared, going to the forensics lab felt like giving up on finding Gerardo alive.
Three months later, the family got a call saying that their DNA matched the remains of a young man. "When we got the call, my first reaction was total disbelief," says Gerardo. "If the results were true it meant that all those months I spent searching for him were in vain."
After a second test with an independent forensics expert, both Alma and Gerardo's worst nightmares came true. The DNA matching confirmed that their son was among four bodies found on a road next to the city's main highway.
Gerardo Jr and his three friends were killed the day after their parents paid the kidnappers the ransom. They were burned alive.
The police found the bodies the following day - but it took 19 months for the bodies to finally be identified. In that time, police buried the young men in an unmarked mass grave inside the city.
After a positive DNA match was made, Gerardo's father went to the grave to recover the remains of his son and finally bury him in a cemetery close to home.
Every Sunday he and his wife visit the grave and bring flowers. It's a ritual that gives some solace. "At least now my son has a place to rest," says Alma, barely holding back the tears. "My son didn't deserve this kind of death."
But that's not how Gerardo feels. He says he hasn't had a restful night since the day his son disappeared.
"I don’t have peace," he says. "Every night I remember what happened from the moment he left the house. I imagine how they grabbed him, threw him in a truck and set him on fire and I wake up."
And the Pe?a family is just one of thousands of families destroyed by Mexico's ongoing violence.