Volunteers who have converged on the Mexican border to watch for illegal immigrants are disrupting U.S. Border Patrol operations by unwittingly tripping sensors that alert agents to possible intruders, an agency spokesman complained Monday. Scores of participants in the Minuteman Project began assembling late last week and planned to begin regular patrols on Monday, in an exercise some law enforcement authorities and civil rights groups fear will result in vigilante violence. Many of the volunteers were recruited over the Internet, and some planned to be armed. Over the past few days, they have set off sensors, forcing agents to respond...