An engineer walks past a cross made of flowers outside the building of the explosion at the headquarters of Mexico's oil giant PEMEX, in Mexico City, capital of Mexico, on Feb. 2, 2013. Mexican and American experts carried out the research on reasons of the powerful explosion at the building E-B2 of the headquarters of Mexican oil giant Pemex, which caused the death of 33 people, injuring 121 others. (Xinhua/Shi Sisi)

Experts work on the debris at the site of an explosion at the headquarters of Mexico's oil giant PEMEX, in Mexico City, capital of Mexico, on Feb. 2, 2013. Mexican and American experts carried out the research on reasons of the powerful explosion at the building E-B2 of the headquarters of Mexican oil giant Pemex, which caused the death of 33 people, injuring 121 others. (Xinhua/Shi Sisi)

An expert works on the debris at the site of an explosion at the headquarters of Mexico's oil giant PEMEX, in Mexico City, capital of Mexico, on Feb. 2, 2013. Mexican and American experts carried out the research on reasons of the powerful explosion at the building E-B2 of the headquarters of Mexican oil giant Pemex, which caused the death of 33 people, injuring 121 others. (Xinhua/Shi Sisi)

Experts work on the debris at the site of an explosion at the headquarters of Mexico's oil giant PEMEX, in Mexico City, capital of Mexico, on Feb. 2, 2013. Mexican and American experts carried out the research on reasons of the powerful explosion at the building E-B2 of the headquarters of Mexican oil giant Pemex, which caused the death of 33 people, injuring 121 others. (Xinhua/Shi Sisi)

Experts work on the debris at the site of an explosion at the headquarters of Mexico's oil giant PEMEX, in Mexico City, capital of Mexico, on Feb. 2, 2013. Mexican and American experts carried out the research on reasons of the powerful explosion at the building E-B2 of the headquarters of Mexican oil giant Pemex, which caused the death of 33 people, injuring 121 others. (Xinhua/Shi Sisi)

Experts work on the debris at the site of an explosion at the headquarters of Mexico's oil giant PEMEX, in Mexico City, capital of Mexico, on Feb. 2, 2013. Mexican and American experts carried out the research on reasons of the powerful explosion at the building E-B2 of the headquarters of Mexican oil giant Pemex, which caused the death of 33 people, injuring 121 others. (Xinhua/Shi Sisi)

An expert takes photos of debris at the site of an explosion at the headquarters of Mexico's oil giant PEMEX, in Mexico City, capital of Mexico, on Feb. 2, 2013. Mexican and American experts carried out the research on reasons of the powerful explosion at the building E-B2 of the headquarters of Mexican oil giant Pemex, which caused the death of 33 people, injuring 121 others. (Xinhua/Shi Sisi)

An expert works on the debris at the site of an explosion at the headquarters of Mexico's oil giant PEMEX, in Mexico City, capital of Mexico, on Feb. 2, 2013. Mexican and American experts carried out the research on reasons of the powerful explosion at the building E-B2 of the headquarters of Mexican oil giant Pemex, which caused the death of 33 people, injuring 121 others. (Xinhua/Shi Sisi)