(CNN) -- Gunmen opened fire and tossed a grenade inside a downtown bar early Saturday in the tourist district of Guadalajara, Mexico, killing six people and wounding another 37, authorities said.
Jalisco State Prosecutor Tomas Coronado Olmos said the attack may have been the result of a fight between people earlier in the night. Some of the men left, but threatened to return, he said.
The attack happened early in the morning just as a flood of people were leaving.
A Colombian and a Venezuelan are among the possible victims, though their nationalities were still under investigation Saturday, the state prosecutor added.
Among the 37 wounded, 26 have light injuries while 11 have serious ones, said Medical Services Director Yalic Nordin.
Violence has escalated in Guadalajara, an economic and tourist hub in Mexico, which in recent weeks has seen a spate of killings and road blockades by organized criminals.
The wave of violence may be related to the dispute between drug cartels after the death of alleged cartel member Ignacio Nacho Coronel, who was killed last July in military operation. It has cast a pall on Guadalajara, picked to host the Pan American Games in October.
The U.S. Consulate General in Guadalajara prohibited U.S. government officials earlier this month from travelling after dark between Guadalajara and the Guadalajara International Airport, and told U.S. citizens there to consider doing the same because of "marked escalation of criminal activity" in the area.
According to the Mexican government, there have been more than 34,600 drug-related deaths since President Felipe Calderon began a crackdown on cartels in December 2006.
Journalist Belen Zapata contributed to this report