MEXICO CITY — The killing of a candidate tipped to become a state governor in Mexico has spiked tensions as election-related violence reached a peak unseen for more than a decade. Immediate suspicion in Monday's shooting of Rodolfo Torre Cantu fell on the country's warring drug gangs since his northeastern state of Tamaulipas, bordering Texas, has seen fierce turf battles in recent months. Mexican President Felipe Calderon denounced it as a "cowardly assassination" which he blamed on organized crime, after an emergency meeting with his security cabinet. Torre, the favorite to win the gubernatorial race from the Institutional Revolutionary Party (PRI), was killed near the Ciudad Victoria airport. Local television showed images of at least four bodies covered with sheets lying on the road next to two cars. It was the most high-profile killing yet in the tense run-up to Sunday's polls that will see 12 governors chosen and elections for local officials in 14 out of 31 states.