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(CNN) -- A day after an explosion rocked its offices in Mexico City, Mexico's state-run oil giant Pemex says it's too early to speculate on the cause of the deadly blast.
At least 33 people -- mostly women -- were killed in the explosion, Pemex chief Emilio Lozoya said at a news conference Friday. More than 100 were injured, including 52 who remain hospitalized.
Lozoya was asked if he believed Pemex, the powerful but often criticized state oil monopoly, was the target of a bombing, but he declined to speculate. Experts from Mexico and abroad are investigating the scene to determine the cause, he said.
Mexican Attorney General Jesus Murillo Karam similarly refused to rule out possibilities.
"We have experts in everything, absolutely everything," he told reporters, adding that investigations are under way to determine whether the blast was the result of an accident or an attack.
Some witnesses have reported smelling a strong odor of gas after the blast.
It is a complicated investigation that "cannot be explained in a few hours," Lozoya said.
The priority, Lozoya said, is providing support to the victims' families and rescuing anyone who may remain trapped.
Mexican President Enrique Pena Nieto visited victims in the hospital Friday, offering condolences and promising help.
The explosion happened Thursday afternoon at an administrative building in the Pemex office complex.
"People were screaming. ... You could see pieces of the wall falling to the ground," said Joaquin Borrell Valenzuela, an attorney for the Pemex comptroller's office, who was in a courtyard outside the building at the time of the blast.
Paramedics quickly arrived and started pulling people from the rubble.
Thousands of people work at the Pemex headquarters, which includes a 54-story building that is nearly 700 feet tall. The annex where the explosion happened is adjacent to the tower.
The oil company's production capabilities have not been harmed, and operations and oil production will continue Friday as normal, Lozoya said.
Employees at the Pemex annex said that the basement of the building, where he explosion is believed to have originated, housed a water treatment facility, storage areas and archives. The first floor, which also was heavily damaged, is where the company's human resources team worked.
One employee, Silvia Hernandez, recounted how she was finishing up her workday at another building and standing in line to clock out when she heard a blast.
"I saw the explosion of glass, people bleeding; it was something very shocking," she said. "I can't say more because the memory makes me nervous. I was terrified, anxious."
A bridge that connected her building with the annex collapsed, likely trapping employees, she said.
Another employee, who declined to give his name for fear of retribution, spoke about his failed attempt to locate his brother within the rubble.
The man smoked cigarette after cigarette in the predawn hours of Friday as he waited at the medical examiner's office to claim his brother's body. More than 100 relatives waited outside the offices with him.
The man said he was among the employees who began rescue efforts in the immediate aftermath of the explosion. He knew his brother worked in the human resources area, which had been damaged, so the man covered his face with a rag and ran there to search for him.
The man said he removed debris and helped rescue others, but never reached his brother. By then, professional rescuers arrived and took over the job of looking for survivors.
CNNMexico's Juan Pablo Mayorga and Arturo Ascención contributed to this report.