MONTERREY, MEXICO (BNO NEWS) -- Mexican President Felipe Calderón on Friday declared three days of national mourning after more than 50 people were killed in an attack on a casino in the industrial city of Monterrey in Nuevo León state.

Calderón said in a nationally televised address before flying to the northern city of Monterrey that the "act of terror and barbarism" that occurred on Thursday afternoon is "the most serious attack against the innocent civilian population the country has seen for a long time". He vowed to use all the government resources to find those responsible for the attack, who he called "terrorists".

Calderón also blamed the United States for the situation in the country and called on the American government "to reflect on the tragedy we are living in in Mexico and many other Latin American countries, in large part from the insatiable consumption of drugs involving millions and millions of Americans."

"Part of the tragedy that we Mexicans are experiencing has to do with being next to the world's largest drug user and also to the world's largest weapons seller, which pays billions and billions of dollars to criminals around the world to supply it with drugs," Calderón added.

Meanwhile, U.S. President Barack Obama also condemned "the barbaric and reprehensible attack" in a statement released by the White House. Obama offered America's condolences to the victims and their families and vowed to continue to assist Mexico in the fight against organized crime.

"The people of Mexico and their government are engaged in a brave fight to disrupt violent transnational criminal organizations that threaten both Mexico and the United States," Obama said. "The United States is and will remain a partner in this fight."

The Mexican attorney general's office is offering a 30 million pesos ($2.4 million) reward for information leading to those behind the attack. Several armed men carrying assault rifles and containers full of gas began shouting and warning everyone within the casino to exit because they were to burn the casino.

According to preliminary reports, most of the victims tried to exit through the casino's emergency exits, but they were reportedly locked. Others ran upstairs fearing an attack from the gunmen, while several more sought for a hiding place in the bathrooms and other areas.

The Governor of Nuevo León, Rodrigo Medina de la Cruz, confirmed the deaths of 53 people, but reports indicate that the death toll could rise. Witnesses said some 150 people were inside the building when the attack occurred, among them visitors and employees.

More than 42,000 people have died in drug-related violence since Mexican President Felipe Calderón began his campaign to fight organized crime in December 2006, according to official figures.