(CNN) -- At least 40 people were killed and numerous others injured in a reported grenade attack at a casino in Monterrey, Mexico, the capital of Nuevo Leon, according to attorney general in that northern state.
The incident occurred around 4 p.m. local time (5 p.m. ET) at the Casino Royale when two people aboard a vehicle arrived, and one threw three grenades into the building.
There were conflicting unconfirmed reports from local media that the assailants poured gasoline on the building before setting it on fire.
Between 20 and 30 people were trapped in the casino because of debris from the explosions, said Cmdr. Angel Flores with the Green Cross.
Video from the scene showed a burned-out building as firefighters made rescue attempts to break the wall of the facade of the casino to release the smoke inside the building.
Mexican President Felipe Calderon sent the following statement from his official Twitter account (translated from Spanish): "With deep consternation, I express my solidarity with Nuevo Leon and the victims of this abhorrent act of terror and barbarism."
He added more: "These reprehensible acts require us all to persevere in the fight against gangs of unscrupulous criminals. All the support to NL (Nuevo Leon)."
About five families were outside the casino waiting for information from authorities, local journalist Javier Estrada reported.
Gustavo Madrazo was one of those waiting outside the casino. He said his wife, Martha, and her sister, Miriam Gonzalez, were inside.
Authorities have not identified any of the victims.
So far, no representative for the company that manages the casino had arrived outside. The Mexican Army and state and municipal police forces were also on the scene.
Alejandro Poire, Mexico's top national security spokesman, said the federal government has made contact with local officials and that Calderon has spoken to the governor of Nuevo Leon to offer support.
Poire said those who carried out the attacks will be held responsible. "They will pay for their crimes. ... We will do absolutely everything ... to restore tranquility."
The National Commission on Human Rights in Mexico sent a news release saying it has opened an investigation regarding the response to the events at the casino.
"The priority is to help safeguard the human rights enshrined in the Constitution of the United Mexican States and international treaties. The commission again makes clear that the insecurity in the country requires public officials responsible for implementing and enforcing the law, carry out preventive actions to protect society and away from any violence."
"The National Commission will monitor the actions taken by the competent authorities to respond adequately to the aggrieved persons."
The NHRC says it has also sent personnel to the scene including physicians, psychologists and lawyers to work jointly with authorities.
Nuevo Leon, in northeastern Mexico, has seen several grenade attacks so far this year. On August 13, four civilians were wounded when an armed group fired two grenades at a jail in the municipality of Apodaca.
In July, gunmen entered a downtown bar in Monterrey and shot 20 people dead. A public safety spokesman told CNN the attack was likely sparked by a dispute between organized crime groups for control of the El Sabino Gordo nightclub, where drugs were allegedly sold.
Nuevo Leon and the neighboring states of Coahuila and Tamaulipas have been the scene of a series of clashes between organized crime groups. The Gulf Cartel and Los Zetas are vying for control of trafficking routes into the United States.
In November 2010, the federal government launched the Coordinated Operation Northeast, which involves sending more security forces to the area to tackle crime.
CNN's Nick Valencia contributed to this report