Reward offered over Mexico casino fire
Mexico's government is offering a 30m peso ($2.4m, £1.47m) reward for information leading to those behind a casino attack which killed over 50.
Several gunmen burst into the building in broad daylight, dousing it with fuel and setting it alight.
Officials suspect organised crime was behind the attack, one of the deadliest since a 2006 crackdown on drug cartels.
US President Barack Obama condemned the attack, reinforcing his commitment to helping Mexico combat criminal groups.
"The people of Mexico and their government are engaged in a brave fight to disrupt violent transnational criminal organisations that threaten both Mexico and the United States," he said in a statement released by the White House.
"The United States is and will remain a partner in this fight."
In a tweet, Mr Calderon said the attack was "an abhorrent act of terror and barbarism" that requires "all of us to persevere in the fight against these unscrupulous criminal bands".
He also declared three days of mourning for the victims of the fire.
The attack happened mid-afternoon when about 100 staff and customers were inside the Casino Royale in the northern Mexican city of Monterrey.
Officials initially said 53 people had died but on Friday the state governor, Rodrigo Medina, lowered this to 52.
One survivor said the gunmen burst in, threatened them and then sprayed some kind of fuel which they set alight.
"It was chaos inside. You couldn't see and the smoke was choking us. Some people didn't reach the exit and fell just metres from the door," said one woman who got out.
Another person who managed to escape said there was complete panic.
"We all wanted to get out at the same time, there was a lot of pushing and we fell to the floor," another woman told Mexican media.
"I just heard explosions and ran to save my life".
Officials said some people had hidden in toilets and offices when they heard explosions instead of trying to get out and had suffocated as smoke engulfed the building.
Some reports suggested the emergency exits had been locked.
State Attorney General Leon Adrian de la Garza said a drug cartel was believed to be behind the attack.
Officials suggested the fire may have been in reprisal for failing to pay extortion money, the Excelsior newspaper reported.
Home to some of Mexico's largest companies, Monterrey and the state of Nuevo Leon have seen rising violence as the Zetas and Gulf cartels vie for control of trafficking routes to the US.
The drug gangs are also increasingly involved in kidnapping and extortion.