It was the sort of scene that plays out thousands of times a day across Mexico City: A tanker truck laden with propane rumbled down a crowded street, pulled up at a hospital and began to pump gas into its storage tanks.

Then something went very wrong. A gas leak led to an explosion Thursday that destroyed a maternity hospital, killing at least three people and injuring more than 70. It's already focusing on attention on Mexico City's unusual reliance on trucking gas through often-chaotic traffic to reach homes and businesses.

"Mexico City is one of the few or maybe the last major capital in the world where gasoline, diesel, fuel oil, all the fuels, propane, are distributed by cargo trucks, tankers and gas cylinders," said Jose Luis Luege, a chemical engineer and former federal environment secretary.

A fleet of 10,000 tank trucks rumble across Mexico filling stationary tanks through hoses and pipes. An additional 14,200 open-bed trucks handle the 20 million gas cylinders in circulation, hawked by venders who slowly make their way through city neighborhoods endlessly shouting "gaaaaaaaas." It's not unusual to see one man pushing a cylinder as tall as he is down the street on a dolly.

According to the propane distributor trade association, Amexgas, there are about 1 million propane deliveries every day in the country.

In the United States, propane tanks and cylinders are dominant in rural areas where the sparse population discourages investment in natural gas pipelines. But in cities, natural gas is more common to fuel homes and businesses.

Dave Komm, a principal engineer at Augspurger Komm Engineering Inc. in Arizona, said natural gas and propane are equally safe if used properly. Potential danger arises in how they are distributed.

"Natural gas usually comes through fixed lines," Komm said. "Those lines are usually underground. Here in the (U.S.) some of them are quite old, maybe 50 years old or more, and so they've begun to deteriorate. Unless they're checked periodically they can be dangerous.

"Conversely, with propane there's a delivery system which involves trucks or cylinders and so you have the human factor where people have to make connections."

Propane, however, tends to sink and pool if there's no breeze or it's in a somewhat enclosed space, he said. It could take as little as a light switch or unplugging something to ignite.

"Once the cloud is out there, if it's not dissipated, it's just a question of time before it ignites."

Witnesses to Thursday morning's explosion at the Maternity and Children's Hospital of Cuajimalpa said the truck arrived like it did every day and parked near the emergency entrance to fill the tanks on the roof above the kitchen.

People soon noticed a strong gas smell and saw a vapor cloud. The truck's crew worked for 15 minutes trying to stop the leak before calling for firefighters and, too late, urging people to get out.

The three crewmen on the Gas Express Nieto truck were hospitalized and in custody. Mexico City Mayor Miguel Angel Mancera said Friday that they could be charged with negligent homicide.

Julio Larrondo, deputy director of Gas Express Nieto, said the company would cooperate with investigators. He said the company, which has served Mexico City hospitals for years, follows all regulations and regularly trains its workers.

The company is one of Mexico's four biggest propane distributors. Each month it makes 70,000 deliveries to stationary tanks and distributes 350,000 gas cylinders.

Mexico's energy secretary, Pedro Joaquin Coldwell, said Friday on Radio Formula that it appeared the company's documents were in order, but he assured that an investigation would find any irregularities.

Meanwhile, overshadowed by the hospital explosion, a worker was killed and two were injured in the neighboring State of Mexico by an explosion Thursday afternoon inside a propane tanker they were painting.