MEXICO CITY — Assassins gunned down a senior police official in the border city of Ciudad Juarez early Saturday as Mexico's gangsters pressed their counteroffensive against the country's security forces. Municipal Police Chief Juan Antonio Roman was shot about 2 a.m. in front of his house on the outskirts of the city, which is across the Rio Grande from El Paso. Another of Roman's police commanders was shot shortly before he was killed. Roman's was one of more than 100 deaths, including those of at least 20 police officers, attributed to organized crime last week across Mexico. Among those killed...