It's part of a nightly bombardment of Border Patrol vehicles and agents all along the southwestern border.
Chris Van Wagenen, a senior patrol agent, says "it's mostly going to be smugglers, guides, gangbangers basically. People who make money off of this.
"This" means smuggling people and drugs into the U.S. It's gotten a lot harder to cross the border, and smugglers are reacting to more fences and more agents with more violence.
"They believe, almost like a terrorist, 'we're going to intimidate you and hurt you until you back off,'" Van Wagenen says.
Assaults against agents in the Yuma sector have shot up to 126 in the first six months of this fiscal year. That's approaching the number of assaults all last year.
Agent Ben Vick was lucky he was wearing a helmet when this brick hit his head.
"I looked down and though, 'Oh, that's what hit me.' Because before that, I had no idea. It was just such a huge blow," Vick says.
With cameras mounted on Agent Vick's vehicle, CBS News set out to see what happens just after dark. A rock quickly hit the car.
"It gives you an idea of what we have to deal with down here. Things being thrown over the fence — heavy things," Vick says.
The Mexican side of the border looks more like a back alley than an international boundary. There's plenty of ammunition for anyone who wants to attack an agent on the other side, and the people who live there say police rarely patrol.
Without more help from Mexico, all agents can do is patrol the border in "war wagons," with windows shielded by steel. The way they see it, as long as there are people desperate to cross into the U.S., there will be smugglers willing to do anything to keep their business.