MEXICO CITY » Injured and bleeding, mothers grasping infants in their arms fled from a maternity hospital shattered by a powerful gas explosion Thursday, and rescuers began smashing sledgehammers through fallen concrete hunting for others who might be trapped.

A nurse and a baby died in the blast and a second infant died Thursday night, Mexico City authorities said. More than 70 people were injured in the blast that collapsed about three-fourths of the hospital, but by late in the day rescuers determined no one was left trapped in the rubble.

Mayor Miguel Angel Mancera said some of the injured were about to be released from area hospitals, including some mothers who suffered injuries while using their bodies to shield their children.

A 25-year-old nurse and a newborn between 2 and 3 weeks old died at the scene and another infant died several hours later at another pediatric hospital, said Armando Ahued, the city’s health secretary. He had said earlier that 21 babies in all had been injured, and nine of those 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 come 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 and gas 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.

People started to evacuate the hospital, and then came a devastating explosion that sent up an enormous fireball and plumes of dust and smoke. Herrera saw injured mothers walking out carrying babies. Officials said 110 people were inside the 35-bed hospital when the truck blew up.

"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.

Margarita Palma of Amexgas, a trade association of Mexico’s propane distributors, said 80 percent of Mexicans use propane rather than natural gas delivered by mains. Liquified propane, which is highly explosive, is distributed to homes and businesses either by trucks like the one that exploded or in cylinders, she said.

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.

Miguel Angel Garcia smoked a cigarette outside Hospital ABC-Santa Fe, trying to calm his nerves while he waited to see his wife and new baby daughter, who had been moved there.

Garcia, 22, had been driving a transit bus when he heard about the explosion at the hospital where his wife had given birth to their second child just the day before. He dropped off his passengers, then his bus and took off for the hospital.

"When I arrived and saw it in pieces, I thought the worst," Garcia said. He waited for an hour before authorities told him his wife and daughter had been taken to the other hospital in the nearby neighborhood of Santa Fe. A nurse there told him both were fine, but he hadn’t been allowed to see them yet.

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

It was the closest hospital to the explosion and received 31 patients, including 17 children.

There were seven babies with serious injuries in intensive care, said Dr. Moises Zielanowski, the hospital’s director of operations, as well as four adults in serious condition. Injuries included burns, fractures and bruises.

He said the hospital was working to identify six of the babies who arrived unaccompanied and without identification.

The gas truck driver and two other employees of the Express Nieto company were hospitalized but were in custody, Mancera said. He said the company has provided gas to all the city’s public hospitals since 2007.

The incident prompted tweets from President Enrique Pena Nieto to Pope Francis, who wrote, "We are praying for the victims of the explosion in Cuajimalpa, Mexico."

Associated Press writers E. Eduardo Castillo, Katherine Corcoran, Emilio Lugo and Christopher Sherman contributed to this report.