U.S. Department of Homeland Security officials say the number of assaults is up this fiscal year after more than doubling between 2004 and 2007, USA Today reported Wednesday.

"We've had occasions of people shooting at agents, trying to run agents down with vehicles, throwing large rocks or pieces of brick or concrete at agents, which can actually be fatal," says Homeland Security Secretary Michael Chertoff.

Chertoff says the escalation is the result of stepped-up enforcement that aims to put smugglers out of business.

Most of the assaults have involved "rockings" in which rocks, bricks and objects are thrown at agents.

Agent Shawn Moran of the Border Patrol in Imperial Beach, Calif., told USA Today the rockings are "like biblical stoning. This is like what they used to do to kill people."

Chertoff was Wednesday headed to Mexico to discuss border issues with Mexican officials.