A nurse who was trying to help newborns to safety as a dangerous gas cloud formed at a Mexico City hospital has died of injuries she suffered when it ignited in a massive fireball, authorities said Wednesday.

Monica Orta Ramirez, 35, became the fourth fatal victim of last week's explosion at the Maternity and Children's Hospital in Cuajimalpa, a borough on the western edge of the capital. She died Tuesday, the city government said in a statement.

The explosion occurred the morning of Jan. 29 when a propane tanker truck arrived to supply fuel to the hospital. Authorities are still investigating but suspect that a valve leak on the truck led to the deadly gas accumulation.

As workers rushed to evacuate the hospital, the cloud ignited, leveling 70 percent of the building.

A second nurse and two babies died previously, and more than 70 people were wounded. Twenty of them remained hospitalized as of Tuesday night, including 9 newborns.