<p>LOS ANGELES — Each year, thousands of illegal immigrants sneak across the border between Tijuana and San Ysidro, Calif.</p>
<p>A thin metal border fence helps separate the United States from Mexico and has helped stem the flow of immigrants but it's not enough. Now, the U.S. Border Patrol is trying to finish building a second fence but the effort is being blocked by the California Coastal Commission (search) because of environmental concerns.</p>