Mexican city's journalists scared silent Rival Mexican cartels employ violence to ensure silence By Hector Tobar Los Angeles Times NUEVO LAREDO, Mexico - Here, it's better not to know. Information can be poison in this border city. When there's a shootout downtown, even the most ambitious radio reporter will not necessarily rush to the scene. So it went the day last month that four undercover federal police officers were ambushed and killed in thick lunch-hour traffic on the city's busiest street. The offices of several newspapers and radio stations were just blocks away - but the news broke 700 miles...