(Via Hotair), a Border Patrol official claims that the aim of the agency is not to stop "illegal immigrants" or illegal narcotics:

Border Patrol agents don't have the responsibility of apprehending illegal immigrants, Carlos X. Carrillo, chief patrol agent for the Laredo sector, said at a town hall meeting Wednesday."The Border Patrol is not equipped to stop illegal immigrants," Carrillo said, noting that illegal narcotics are also not on the agents priority list. "The Border Patrol mission is not to do any of those things," he emphasized.

The Border Patrols mission is to keep the country safe from terrorist and terrorist weapons, he said. Carrillo added that when and if terrorists come into the country, the agents will be ready.

This history of the Border Patrol certainly draws attention to the BP's interest in stopping "illegal" crossings, and this paragraph on the BP today references "illegal" crossers and illicit drugs:

New dangers confront our nation's borders. As part of Customs and Border Protection, the Border Patrol is taking on the additional challenge of protecting our borders against terrorism. History has shown that it matters little whether the drug is alcohol or marijuana; the illegal migrants, Chinese or Mexican;
or the terrorists, hijackers, or suicide bombers - in any scenario, we can count on the Border Patrol to protect us from threats to the safety and security of our borders.

The Border Patrol's mission statement also says that the organization will "steadfastly enforce the laws of the United States."