At least 33 are dead and more than 120 injured after an explosion in one of Mexico's tallest buildings, the headquarters of Petroleos Mexicanos — or Pemex — the Mexican state oil company.
There were reports of many more still trapped at the 51-story tower in Mexico City, according to MyFoxLA.com.
According to the Associated Press, the cause of the blast is still unclear.
"It seems like, from what experts can observe, is that it was an accident," Pemex Director-General Emilio Lozoya was quoted by the AP as saying on local television. "However no line of investigation will be discounted."
Mexican President Enrique Pena Nieto urged people not to speculate about what my have caused it. Some theories include an electrical fire, faulty air conditioning and a possible attack.
"We will work exhaustively to investigate exactly what took place, and if there are people responsible, to apply the force of the law on them," Pena Nieto told reporters, Reuters reported.
Pemex said on its Twitter account this afternoon that the death toll had risen to 33 as the country declared three days of mourning.
Britain's Daily Mail wrote that rescuers were using dogs, trucks with mounted lights and an oil company crane to search for survivors late Thursday.
The paper cited Interior Minister Miguel Osorio Chong as saying it was uncertain whether there were people still trapped.
MyFoxLA wrote that Pemex had a poor safety record, with dozens of accidents and explosions claiming hundreds of lives and injuring thousands over a period of decades.
However, there were reports that the blast may have occurred in the basement garage of an administrative building adjacent to the iconic Pemex building.
The Associated Press cited employee Cristian Obele as telling Milenio television:
"It was an explosion, a shock, the lights went out and suddenly there was a lot of debris. Co-workers helped us get out of the building."
The AP cited the company as saying on Twitter that experts from the Attorney General's Office were analyzing the explosion and any reports of a cause were speculation.
The incident comes as Pena Nieto pushes to open the oil giant to more private and foreign investment and analysts speculated that the latest tragedy could help his case.
“So what will the Pemex explosion mean for the national debate on energy reform? It puts Pemex firmly in the spotlight for a start,” Duncan Wood, director of the Mexico Institute at the Woodrow Wilson centre in Washington, said on his Twitter account, the Financial Times reported.
“Pemex needs to be modernised from top to bottom, from exploration and production to basic practices ... Will legislators [now] recognise that Pemex has fallen behind the times?”