Injured and bleeding, mothers carrying infants fled from a maternity hospital shattered by a powerful gas explosion Thursday, and rescuers swung sledgehammers to break through fallen concrete hunting for others who might be trapped.

At least two people were killed and more than 60 injured, Mexico City Mayor Miguel Angel Mancera said at a news conference. The known dead were a woman and a child. Officials earlier said at least four people had been killed.

About 75 percent of the hospital collapsed, officials said, and the priority was to continue digging in search of any trapped survivors. Authorities said they had confirmed that none of the children registered in the hospital were missing, but said it was possible that others who had come for appointments could be beneath the rubble.

The city’s health secretary, Armando Ahued, said the adult victim was a 25-year-old woman and the child was between 2 and 3 weeks old. He said 21 babies were injured. Seven infants and seven adults were in serious condition after being rushed to other hospitals.

Thirty-five-year-old Felicitas Hernandez wept as she frantically questioned people outside the wrecked building, hoping for word of her month-old baby, who had been hospitalized since birth with respiratory problems.

“They wouldn’t let me sleep with him,” said Hernandez, who had brought the child to the city-run Maternity and Children’s Hospital of Cuajimalpa because she had no money. Later, authorities told her to check at another hospital where she reported finding her baby uninjured.

The explosion occurred at 7:05 a.m. when a tanker truck was making a routine delivery of gas to the hospital kitchen started to leak. Witnesses said the tanker workers struggled frantically for 15 or 20 minutes to repair the leak while a large cloud of gas formed.

“The hose broke. The two gas workers tried to stop it, but they were very nervous. They yelled for people to get out,” said Laura Diaz Pacheco, a laboratory technician.

“Everyone’s initial reaction was to go inside, away from the gas,” she added. “Maybe as many as 10 of us were able to get out. … The rest stayed inside.”

Workers on the truck yelled: “Call the firefighters, call the firefighters!” said anesthesiologist Agustin Herrera.

As people started to evacuate the hospital, there was a devastating explosion that sent up an enormous fireball and plumes of dust and smoke. Herrera saw injured mothers walking out of the damaged building, carrying babies.

“We avoided a much bigger tragedy because the oxygen tanks right beside (the area) didn’t explode,” Herrera said.

The worst hit parts of the hospital were the neonatology, reception and emergency reception units, he said.

Officials said 110 people were inside the 35-bed hospital at the time, and there was no indication by late afternoon whether any were unaccounted for.

“We can’t rule out anything about people being trapped under the rubble,” said Fausto Lugo, Civil Protection secretary.

Homes next to the hospital had broken and cracked windows and fallen shingles from the blast, and many neighbors ran to help evacuate victims from the debris, local resident Carlos Soria Rezendiz said.

As the day wore on, people arrived at Hospital ABC, the closest to the explosion site, offering diapers and baby formula. There was an hour-long wait to donate blood.