For the Obama White House, pondering how to reshape the Bush administration's war on drugs, the concerns presented by the deepening crisis in Mexico are twofold.

The first was highlighted by the chief of staff, Rahm Emanuel, talking about US-Mexican counter-narcotics co-operation. "They want to clearly stop the guns from the United States going south. We want to stop the drugs coming north," he said.

The second concern is about another equally pernicious commodity migrating north: the violence. The announcement last month that 730 people had been arrested across the US following a 21-month investigation into Mexico's Sinaloa drug cartel confirms suspicions that the cartels are taking root in the US.

A succession of drug-related incidents has fuelled concerns. In January a man was kidnapped outside his home in suburban Phoenix, Arizona. Initially, his family denied anything was wrong. Later, they admitted he was involved in a drugs deal gone sour and his captors were demanding $150,000 (£106,000) .

Within two days he had been freed with the help of Arizona's recently formed kidnap unit, set up in response to the increase in ransom cases. Last year Phoenix police received 366 kidnap-for-ransom reports, a figure they estimate represents half of the true total.