HERMOSILLO, Mexico - As the day care swiftly filled with smoke, caretakers, neighbors and parents fought to evacuate 142 children - many of them babies and toddlers - through a single working exit until rescue crews arrived.

No fire alarm or sprinkler system had gone off, and one mother said a second door to the day care was bolted shut and nobody could find the key.

Forty children were killed in the devastating fire, which raised doubts about safety standards at more than 1,500 centers where Mexico's government provides low-cost care for at least 200,000 children.

President Felipe Calderon, who visited some of the 33 children hospitalized on Saturday night, pledged a thorough investigation into the cause of a tragedy that has stunned Mexico. More than 5,000 attended a Mass for the children at a concert hall yesterday evening in Hermosillo, the capital of the northwestern state of Sonora where the fire erupted Friday. *