Mexicans have endured plenty of horrific crimes during their country's bloody five-year war against drug gangs -- bodies hanging from overpasses, beheadings, mass slayings of migrants and gunfights on crowded streets.
But the torching of the casino that killed at least 52 people was a shocking new low for many.
In a nationally televised speech, an angry President Felipe Calderon declared three days of mourning and labelled the attack on the Casino Royale in Monterrey the worst against civilians in the nation's recent history.
"We are not confronting common criminals," he said. "We are facing true terrorists who have gone beyond all limits."
The attack was different than others in recent years in that the victims weren't cartel foot soldiers or migrants resisting forced recruitment by gangs.
They were part of the middle class, working or gambling in an affluent part of a city that was once considered one of Mexico's safest.
"The media impact that this has is greater, because we're talking about an attack on a civilian population of a certain income," said Jorge Chabat, an expert in safety and drug trafficking.
"Because who was there was from the middle class, the upper middle class of an important city in Mexico."
As the country took in the grisly details of the attack, some said a new, macabre milestone had been reached in a conflict that's claimed nearly 40,000 lives since Mr Calderon launched a drug offensive in December 2006.
A surveillance tape showed eight or nine men arriving in four cars at the casino and setting fire to the building within minutes. The gunmen had ordered people to leave before setting the fire, but many fled further inside.
Officials said they probably died quickly, the majority from smoke inhalation.