MEXICO CITY - Mexican President Enrique Pena Nieto on Friday decreed three days of national mourning in honor of the 33 people who died following a powerful explosion at the headquarters of Mexican oil giant Petroleos Mexicanos (Pemex).

"This tragedy has filled us with sadness, so Mexico is grieving, so I decided to declare three days of national mourning. The national government will support the bereaved relatives at all times, with all they need, "said Pena Nieto while attending a ceremony.

Earlier in the day, Mexican Interior Minister Miguel Angel Osorio Chong announced that the latest death toll had risen to 33 after rescuers concluded their search in the rubble.

The president announced that he has instructed Osorio Chong to carry out a thorough investigation and that the public can know what had caused the explosion at Pemex Tower.

"I have instructed him to have a deep and thorough investigation to clarify the causes of this incident, which will prevent a repeat of a tragedy like it and will not discard any hypothesis," said the president.

He reiterated his most sincere condolences to the families of those who sadly lost their lives and his great sympathy to the people who were injured.

The explosion occurred Thursday at around 3:55 pm local time (1955 GMT) in the lower floors of the 211-meter Pemex Tower in downtown Mexico City. More than 100 people were injured.

Authorities estimated that about 250 people worked on those floors and 10,000 in the headquarters building complex oil. They noted that 1,700 people work in the building.

Pemex was created in 1938 and today has about 150,000 employees. In 2011, the Mexican oil giant earned more than 111 billion U.S. dollars in sales.