It is certainly possible that they targeted him because he was a BP agent, and they may have intended to kill his entire family.
That would be a typical M.O. for a Mexican drug gang. We have all seen the recent articles about gang shootings of Mexican reporters for daring to write stories about them.
Apart from the terrible injustice to the agents themselves, it's another reason why the Ramos/Campeon prosecution was a huge mistake. Why should the drug gangs fear American law enforcement when the President himself appears to be on their side?