Surveillance video captured the suspects making their getaway from the Casino Royale after setting it aflame.
Rescue workers continued digging through the rubble, trying to recover the bodies of the dead. The victims were people working or gambling at the Casino Royale.
Witnesses say the attackers told people to get out, but many of them were terrified and moved further inside the building, becoming trapped.
The attack made a particularly strong national impact because many of the victims belonged to a middle class so far mostly untouched by drug war carnage, said Jorge Chabat, an expert in safety and drug trafficking at the Center for Research and Teaching in Economics.
Mexican President Felipe Calderon declared three days of mourning for the victims, describing the incident as the worst attack on innocent civilians in recent memory.
"We are not confronting common criminals," the visibly angry president said. "We are facing true terrorists who have gone beyond all limits."
The attack even drew the condemnation of President Barack Obama, who called it "barbaric and reprehensible" in a statement.
The Casino Royale had been attacked twice before, including in May when gunmen strafed it from the outside.
The Associated Press contributed to this report.
