Law enforcement officials on both sides of the border agree that the United States is the source for most of the guns used in the violent drug cartel war in Mexico.

“The key thing is to keep improving on our interdiction of the weapons before they even get in there,” said Janet Napolitano, the secretary of homeland security and the former governor of Arizona, who will be testifying before Congress on Wednesday.

Familiar Signs

Sergeant Azuelo quickly began to suspect that the pistol whipping he was investigating was linked to a drug dispute. Within minutes, his detectives had found a blood-spattered scale, marijuana buds and leaves and a bundle of cellophane wrap used in packing marijuana.

Most often, police officials say, the invasions result from an unpaid debt, sometimes involving as little as a few thousand dollars. But simple greed can be at work, too: one set of criminals learns of a drug load, then “rips” it and sells it.

“The amount of violence has drastically increased in the last 6 to 12 months, especially in the area of home invasions, “ said Lt. Michael O’Connor of the Pima County Sheriff’s Department here. “The people we have arrested, a high percentage are from Mexico.”

The violence in the United States does not compare with what is happening in Mexico, where the cartels have been thriving for years. Forbes recently listed one of Mexico’s most notorious kingpins, Joaquin Guzmán, on its list of the world’s billionaires. (No. 701, out of 793, with a fortune worth $1 billion, the magazine said.)

But a crackdown begun more than two years ago by President Felipe Calderón, coupled with feuds over turf and control of the organizations, has set off an unprecedented wave of killings in Mexico. More than 7,000 people, most of them connected to the drug trade or law enforcement, have died since January 2008. Many of the victims were tortured. Beheadings have become common.