The death of Rodolfo Torre Cantu, candidate for the Institutional Revolutionary Party, was blamed on organized crime and called one of the most high-profile political assassinations in Mexico since the 1994 killing of PRI presidential candidate Luis Donaldo Colosio.
Torre's brother, Egidio Torre Cantu, replaced him on the ballot and won, as the state became a battleground between the Zetas and Gulf cartels.
Epidemic of fear
As the conflict dragged on, daily life in many parts of Mexico took on an edginess over potential attacks. Mexicans became increasingly afraid to go out at night, and their confidence in authorities dropped year by year.
In May 2010, popping sounds heard at a concert venue near Monterrey led to a stampede that left five people dead.
In May 2011, a video circulated of a Monterrey schoolteacher leading kindergarteners in song as they duck for cover from a shootout nearby. The clip of teacher Martha Rivera Alanis showing bravery and poise in the incident moved television viewers and also led to the greater cries of outrage at the government over the growing reach of violence.
In another dramatic moment caught on tape, in August 2011, a shooting outside a professional soccer match in the city of Torreon was heard in the stands and on the field, causing a panicked scene as fans and players ducked for cover or streamed onto the field to escape. No one was killed or injured.
San Fernando migrants massacred
The transnational aspects of Mexico's conflict with drug gangs were laid painfully bare in August 2010, when news emerged that a house near San Fernando, Tamaulipas, had been discovered where 72 kidnapped migrants from Central and South America had been executed.
The massacre, blamed on the Zetas, by then also trafficking in humans, was condemned by the global community and met with embarrassment in Mexican society. The sole survivor of the massacre, 18-year-old Luis Freddy Lala Pomavilla, returned to his native Ecuador under heavy guard.
The San Fernando massacre highlighted the dangers migrants face in their attempt to cross Mexico and reach the United States. An estimated 10,000 migrants from Central and South America have gone missing in Mexico, the national human rights commission says.
Death at the Casino Royale
On Aug 25, 2011, gunmen linked to the Zetas stormed into the Casino Royale gambling club in Monterrey and set fire to it. When the smoke cleared, 53 people were dead, an attack that horrified the population.
Calderon, his wife and top administration officials traveled to Monterrey, dressed in black, and stood before a wreath in honor of the victims. It seemed to matter little to the grieving city when arrests were made in the case. The burned-out shell of the Casino Royale remains standing.Second interior minister killed in aviation crash
On Nov. 11, 2011, Calderon's government suffered a second loss of its second-in-command when Interior Minister Francisco Blake Mora and seven others were killed in a helicopter crash outside Mexico City.
There was no sign of foul play in the crash. A week earlier, Blake had mentioned the 2008 Learjet crashed that killed Mouriño in a Twitter message. The message turned out to be his last.
Reporters become victims
Dozens of journalists have died during the Calderon term. Although counts differ because the professional ties of the journalists at the times of their death have varied (see this map), Mexico is now considered one of the most dangerous places in the world to report the news.
The string of reporters killed in Veracruz state in the last year came to typify the impunity surrounding the deaths of journalists nationwide. When an arrest was made in the April 2012 death of investigative reporter Regina Martinez, few of her surviving colleagues said they were convinced justice had been served.
The incoming government of President-elect Enrique Peña Nieto has said it will adjust Calderon's strategy and focus on reducing the homicide rate. During the presidential campaign, Peña Nieto hired the former director of Colombia's national police as a security advisor.
A poll released Friday said Mexicans were looking to the transition with measured optimism, with 31% saying Peña Nieto would govern better than Calderon, 22% saying worse, and 22% saying they did not know.
Calderon delivered a final videotaped message to the nation that aired Wednesday night. The video, shot in warm-colored soft filters as Calderon is seen signing a letter at a desk and saluting a flag, ends with him bidding farewell. "Thank you very much, and so long, Mexico!" he says. Calderon begins a research and teaching fellowship at Harvard University in January.