Authorities rescued a newborn from a trash bin behind a restaurant Thursday and police said the baby's 22-year-old mother was under investigation.

At first, rescuers thought the baby was dead. Then they heard a faint cry. The infant, born about 10 weeks prematurely, was in critical condition at a St. Louis hospital.

Police Chief Tim Swope said his office was contacted by a hospital nurse, who had just learned from the mother and relatives that the child had been left in a trash bin.

An emergency medical technician found the baby with the placenta and umbilical cord still attached. Todd Ferring said he assumed the baby was stillborn as he gave her to another EMT, but the men saw the baby take a breath, then utter a faint cry.

Police believe the mother gave birth at a home near the trash bin. It wasn't clear how long the child had been abandoned, but Ferring said it was likely several hours.

The mother apparently drove herself or had someone drive her to the hospital, where she was being treated for postpartum symptoms, Swope said. Police were not immediately able to interview her.

Swope would not speculate whether the mother knew the child was alive, or whether she thought it was stillborn. He said the father was not believed to have been involved.

Relatives were cooperative, Swope said. Police hope video surveillance footage from the restaurant will offer additional clues.