MEXICO CITY — His voice cracking with emotion, President Felipe Calderon said Friday that the United States bore some blame for “an act of terror” by gangsters who doused a casino with gasoline and set a blaze that killed at least 52 people.

The attack Thursday in Monterrey, an industrial city of 4 million barely a two-hour drive from Texas, stunned Mexicans and seemed likely to mark a watershed in the country's intensifying war against criminal syndicates.

In a 20-minute televised address to the nation, Calderon gave an unusually blunt assessment of the causes of Mexico’s surging violence before flying to Monterrey to place a wreath at the burned-out hulk of the Casino Royale.

He referred repeatedly to the attack as a terrorist act, elevating the conflict to a new level, at least linguistically, and casting it in terms of a broader struggle for control of Mexico. He said rampant corruption within his nation’s judiciary and law enforcement bore some blame.

But in unprecedented, direct criticism of the United States, Calderon said lax U.S. gun laws and high demand for drugs stoked his nation’s violence. He appealed to U.S. citizens “to reflect on the tragedy that we are living through in Mexico.”

“We are neighbors, allies and friends. But you, too, are responsible. This is my message,” Calderon said.

He called on the United States to “once and for all stop the criminal sale of high-powered weapons and assault rifles to criminals that operate in Mexico.”

Calderon declared three days of national mourning.

The motive of Thursday’s attack wasn't clear, but authorities indicated that it might have been part of an extortion campaign against one of many casinos that operate in Mexico on the margins of the law.

Calderon's blast at the United States underscored frustrations here that there's little appreciation north of the border for the role Americans have played in strengthening the cartels that are responsible for the grisly violence that's claimed as many as 40,000 lives in the last five years.

With weapons bought in the United States, the gangs, whose roots lie in drug smuggling but which have branched out into a variety of criminal enterprises, are better armed than the police tasked with combating them. While Calderon's government has captured dozens of mid- and upper-level gangsters, beheadings, public executions and kidnappings are epidemic, and many Mexicans feel less safe than ever.

“Part of the tragedy that we Mexicans are living through has to do with the fact that we are next to the world’s greatest drug consumer,” Calderon said in his speech, “and also the greatest global arms vendor that pays billions of dollars each year to criminals.”

In a statement, President Barack Obama condemned “the barbaric and reprehensible attack” and lauded Mexico’s “brave fight to disrupt transnational criminal organizations that threaten both Mexico and the United States.”

Of the 52 who died in Thursday's firebombing, 35 were women, mostly in their 40s, 50s and 60s, who were passing time in the casino on a weekday afternoon, civil defense officials said. Ten people were injured in the blaze.

A video taken by a closed-circuit camera that overlooks the casino's entrance showed that the attack unfolded in only two and a half minutes. Four vehicles can be seen pulling into the driveway of the Casino Royale, on San Jeronimo Avenue in a posh area of western Monterrey, at 3:48 p.m. Gunmen jump out of the cars and enter the casino, carrying three canisters apparently filled with gasoline.