Police scoured the snowcapped mountains of northern Colombia Tuesday for a leftist rebel unit blamed for a roadside bomb attack on a government convoy that killed at least 15 elite counterinsurgency troops. The bombing was the latest in a stepped up campaign of attacks this year by the Revolutionary Armed Forces of Colombia, or FARC, apparently aimed at undermining hard-line President Alvaro Uribe's re-election hopes. The rebels detonated a powerful explosive charge as the police convoy drove along a dirt road late Monday in the Sierra Nevada mountains that rise up from the Caribbean coast, said Gen. Alberto Ruiz, operations chief for the national police. Officials initially put the death toll at 11, but Ruiz told The Associated Press in a telephone interview Tuesday that at least 15 members of the elite unit died.