By Duncan Kennedy
BBC News, Mexico City
So far this year more than 1,400 people have died in Mexico as drug cartels battle one another for control of the illegal drugs trade to the US, and battle the authorities trying to stop them.
Gun smuggling from the US to Mexico pays well, says this man
The dead include more than 400 police officers and other public officials.
This follows 2,500 deaths last year and the same number in 2006.
Estimates by the US Drug Enforcement Agency suggest the drugs trade is worth $20bn (£10.3bn) a year.
US law enforcement agencies are now heavily involved in trying to help the Mexicans control the violence.
But Americans are playing another role in the conflict.
More than 90% of weapons used by the cartels originate in the US and there is evidence US citizens are joining the gangs to act as hitmen and women.
We went to a gun shop in the US that had sold a weapon subsequently found in Mexico during an anti-drugs operation.
The owner was shocked when told that the police had traced it via its serial number to his store.
"I'm surprised," he said, "But it's people that kill people, not guns."
It is also people that smuggle the guns into Mexico.
Rocks, not guns
We were introduced to one such person, an American who would only talk if his identity was concealed.
"I can sell guns for two or three times what you pay for them in a shop," he said. "If I don't do it, someone else will. That's the bottom line."
Bottom lines, not lives, are what is driving a market that the smuggler says is being fed by many eager Americans.
Seizures of weapons give an idea of how well-armed the cartels are
But while American individuals are profiting from Mexico's misery, the US authorities are working to halt the flow of weapons.
The Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) has launched Operation Gunrunner, a joint intelligence-gathering enterprise with the Mexican government.
"I think we can make a difference," says Dewey Webb, a veteran ATF agent with 32 years of service.
"I'll know it's a success when we see the cartels throwing rocks at each other, not shooting themselves with guns," he said.
Rosalio Reta's armoury has never included rocks.
Reta is 18 years old, American, and a cartel hitman.
Detectives believe Reta, who is serving 40 years in a US jail for murder, has killed 30 people.
The cartels use people like Reta to enforce their business in the US. Their American nationality means they can cross the border with ease.
Reta is one of at least 15 US citizens used by the cartels, according to police in Texas.
Mexico has intensified its response to the cartels' challenge
Their story has another twist.
These hitmen have received training at a variety of camps along the border with the US.
Reta told police he was in a camp for six months.
"He received basic training, just like in the regular army," says one detective we spoke to, who did not want his identity revealed after receiving death threats.
"Not only would it be training in weapons use and explosives, but it would also be physical training like sit-ups and running," he said.
"Women, too, are going into these camps".
The detective said that he has a very reliable female informant whom he has used for years. She told him she went into a camp for women only.
"She received the same kind of training as Reta," he said. "That meant training in the use of guns, car chases and how to make small explosive devices, all whilst wearing military fatigues."
The detective explained how some of the cartel members are former special forces troops from the Mexican Army, known as Zetas.
"They train because they were trained," he said.
After weeks of research, we decided to visit one of these camps, although most people advised against it.
We were reminded, time and again, that at least 11 journalists have been killed in Mexico by the cartels for trying to cover the story.
But after persuading one federal police force in the north of the country to provide us with an armed escort, we went ahead.
We set out early in the morning, and drove 70km (43 miles) into the desert.
Roads gave way to tracks, confidence gave way to nervousness.
Eventually we pulled up at the gates of one ranch where our escorts said they would go no further.
We climbed over a fence and went on alone, walking for about half an hour across scrub and open desert.
We had been told that the cartels do not stay long in one place and that this camp was regularly swept by the army to make sure they do not return.
We saw tyre tracks, an empty beer can and a recently used open fire. The place had an eerie sense of isolation - ideal, of course, for noisy weapons training.
After less than an hour, we left.
Another police force showed us their footage. In the bathroom of a normal looking house, there was a sink under which they discovered a hidden passage that led to a soundproofed underground room.
At one end, four targets had been erected. In front of them were dozens of high-powered rifles and the ammunition to go with them.
Seeing such stocks, it is no wonder so many police and army units complain about being outgunned by the cartels.
In a different sense, the cartels are also trying to outwit the authorities when it comes to moving their drugs.
We were shown a series of extraordinary films and photos of the cartels' latest piece of apparatus: semi-submersibles.
US Customs have come across at least four of these craft. With up to four crew, these vessels have equipment to evade radar and can travel at up to 20 knots.
Semi-submersibles are one of the many ways the traffickers move drugs
One was found to contain four tons of cocaine.
The gangs often try to scuttle the craft if they are about to be captured. That makes prosecutions harder, though there is talk in the US of a change in law to make the use of these kinds of boats illegal.
If this sounds like the cartels are winning, it is not the whole truth.
While they are still moving large quantities of drugs and are still prepared to kill lots of people, thereby giving the impression of strength and superiority, they are also suffering setbacks.
The Mexican government says the fact that there is so much violence is a sign the gangs are on the defensive, reeling from the pressure being inflicted by the authorities and by their own internal infighting as their leaders are arrested.
Some 25,000 troops are now deployed around Mexico to try to break the cartels.
There have been several high-profile operations that have detained the heads of the Sinaloa Cartel and the Tijuana Cartel.
A banner apparently signed by the Zetas offers soldiers good pay to join
But key figures, like Joaquin "Shorty" Guzman, still evade capture and fuel the cartels' sense of invincibility.
The cartels' brazen activities extend to dangling full-size banners from road bridges urging people to join them. These audacious advertising posters even include a phone number for potential recruits to call.
Their ability to infiltrate local, city and federal police forces to get access to high-ranking officers in order to murder them, is another chilling reminder they are not yet a spent force.
"Something has to be done," said Nora Del Rio, whose husband Juan Antonio was the chief of police in the northern city of Juarez.
"It cannot go on like this," she said. "We are living in terror because of these gangs."
Mexican President Felipe Calderon is in the process of working out an even more comprehensive deal with the US to try to bring the violence to an end.
Known as the Merida Initiative, it has the full support of President George W Bush, although the US Congress has been attaching strings to the $1bn in funding that is due to flow in the next three years.
The experience of Colombia in the 1980s and 1990s suggests the cartels can be subdued, if not beaten.
But for now, Mexico's killing machines, in the form of their assassins and their ceaseless supply of weapons and ruthless ingenuity, do not seem ready to stand, or be pushed, aside.