In an exclusive interview with the BBC's John Simpson, Mexican President Felipe Calderon has warned that corruption among American officials may be making it harder to deal with drug-trafficking between Mexico and the US.
Mexican President Felipe Calderon tells John Simpson about US 'complicity'
Speaking in Mexico City before leaving for his state visit to London immediately ahead of next week's G20 summit, Mr Calderon said violence in the border city of Juarez had fallen by 73% in the month since he sent 7,000 extra troops there.
There has been open warfare in Juarez for more than a year; last year, 5,600 people were killed in drug-related attacks in Mexico, many in Juarez.
The drop in murders there has been a big success for him, especially coming as it does immediately before he meets other world leaders in London.
But analysts in Mexico City point out that there was also a fall in the violence in Juarez last year when he sent a first detachment of troops in. It quickly rose again.
Juarez has become a battleground because the heads of the Sinaloa drugs cartel decided to move in on the lucrative cross-border routes run by the Juarez cartel. The city lies just across the border from the American city of El Paso, in Texas; the two are less than a mile apart.
The riches of the drugs trade have spawned a bloody conflict in Mexico
President Calderon said it was impossible to smuggle tonnes of cocaine into the United States without the complicity of some American authorities.
"There is trafficking in Mexico because there is corruption in Mexico," he told the BBC.
"But by the same argument if there is trafficking in the United States it is because there is some corruption in the United States... It is impossible to pass tonnes of cocaine to the United States without the complicity of some American authorities."
President Calderon complained about the smuggling of American guns into Mexico. Ninety per cent of the guns used in the current wave of drugs violence came from the US, he said, and he pointed out that there were 11,000 gun stores in towns and cities close to the Mexican border.
Relations with the Obama administration in Washington have improved greatly since Hillary Clinton, the American secretary of state, visited Mexico City last week.
She made it absolutely clear that Washington did not regard Mexico as a potentially failed state, as some American officials had previously suggested.
But there is no doubt that the situation along the border is very sensitive politically, and although the White House may wonder privately whether there is some corruption among some American customs, immigration and police officials, it is unlikely to admit it publicly.
As for the border city of Juarez, it is starting slowly to come back to life, now that the army patrol the streets. But at night it is a different story.
The bodies of victims of the drugs war crowd a morgue in Juarez
"No-one here goes out after seven in the evening," a man told us, and we found it was true. By nine o'clock, the streets of the city centre were entirely deserted.
In the past, Juarez was a late-night city. Now most of its shops, bars and restaurants close from the late afternoon onwards.
In a usually popular bar we went into, there were only three customers. Round the corner, a big restaurant was almost entirely empty, the waitresses standing round talking to each other in a corner.
The sound of gunfire from those weapons smuggled across the border is much rarer in Juarez now than it was four weeks ago.
But public confidence will take much longer to return. Especially at night-time.
John Simpson's full interview with President Calderon will air on BBC World TV at 2030 and 2230GMT on Monday.
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