Mexico has arrested five suspected drug gang members in connection with the burning of a casino last week that killed at least 52 people, one of the worst attacks on civilians in the country in years.

Thursday's arson attack on the upmarket casino in the northern city of Monterrey has deepened skepticism about President Felipe Calderon's fight against drug cartels, putting new pressure on his embattled government to root out crime.

Rodrigo Medina, governor of Nuevo Leon, where Monterrey is the capital, said Monday the five suspects arrested had confessed to involvement in the killing and that the hunt for other perpetrators continued.

From the initial indications we've observed the target was the casino, not the civilian population, Medina told a televised news conference, adding that a strong line of investigation was that the casino was being extorted.

Medina said the suspects identified themselves as members of the Zetas drug cartel, which has ravaged the state and other parts of Mexico with killing, kidnapping and extortion.

Calderon declared three days of national mourning after the attack and in a television interview Monday pledged to continue the fight against organized crime.

He admitted corruption among judges and police was still helping criminal gangs go unpunished.

We have to speed things up because we know the criminals have corrupted the institutions at all levels of government, Calderon said. This cannot be tolerated.

Violence has soared in prosperous Monterrey since Medina became governor and hundreds of demonstrators staged a protest outside his office on Sunday, calling on him to resign.

Calderon sent hundreds of extra troops to Monterrey after the attack, which was perhaps the worst yet to directly affect the kind of middle-class voters that form the backbone of his conservative National Action Party, or PAN.

Voter surveys show the drug war has undermined support for the PAN ahead of next July's presidential election, and latest polls show the party is likely to be ousted from office.

The situation in Nuevo Leon has coincided with a surge in violence across Mexico since Calderon sent in the army to crush the powerful cartels when he took office in December 2006.

Calderon pointed a finger at the United States again, demanding the U.S. Congress curb illegal weapons sales across the Mexican border and stamp out the demand for drugs that he said was fueling the violence in his country.

It's not just one or two people. There are more than 100 million Americans who use or have used drugs, Calderon said. And they are responsible.

Monterrey was once seen as a paragon of economic development in Latin America. But the escalating violence has sparked concerns that both the city and the country are losing the fight against organized crime.

The Mexican Attorney General's office on Friday offered a reward of 30 million pesos ($2.4 million) for information leading to the arrest of perpetrators of the casino attack.

(Reporting by Dave Graham; Editing by Jackie Frank and Bill Trott)