Mexican authorities denied Thursday that a girl was trapped inside the rubble of a school that collapsed in a powerful earthquake, highlighting the confusion that still reigns two days on.

"We have carried out a full count with the directors of the school and we are sure that all the children are either safe at home, in the hospital or unfortunately died," Angel Enrique Sarmiento, a top officer in the Mexican marines, told journalists at the ruins of the Enrique Rebsamen school in Mexico City.

"There are indications there may be an (adult) still alive in the rubble. There are traces of blood... as if the person had dragged him or herself and may still be alive," he added.

Rescue workers had told journalists they were certain a girl was alive beneath the rubble, but the different versions of the story varied widely.

The story made headlines around the world after the quake, injecting a ray of hope into a tragedy that killed 19 children at the school and more than 200 people nationwide.

Some rescue workers gave vivid details of supposed interactions with the girl beneath the mangled remains of the school.

"We know that there is a child alive inside. What we do not know is how to reach her... without risking a collapse and putting rescuers in danger," rescue coordinator Jose Luis Vergara had told TV network Televisa.

Authorities had reported that a slightly-built civilian volunteer was able to squeeze into a narrow passage through the rubble to pass the girl oxygen and water through a tube.

Vergara said the girl had spoken, managing to say "I'm very tired."

Some of the hundreds of volunteer emergency workers at the scene were heartbroken that the story turned out to be false.

"I was completely sure that there was a girl under the rubble. I was bringing smelling salts and oxygen tanks to the exact spot where the rescue workers said she was trapped," said Aracely Suarez, a 23-year-old economics student whose face was reddened by the sun after two days working at the site.

"We were all giving everything we had to rescue that girl, all the children," she told AFP.

But the search went on for the adult who may be trapped at the site, and some 200 missing people around the capital.