Mexican officials said Monday that two worn-out bolts on a gas tanker truck broke, causing a leak that resulted in a hospital explosion that killed five people last month.

The results of the investigation again highlighted the unsafe conditions that many propane tankers operate under.

The failure "was caused by (metal) fatigue on the bolts due to a lack of proper maintenance," said Mexico City chief prosecutor Rodolfo Rios.

When the two bolts on a pipe flange connected to the truck's pump cracked, a gasket partially blew out, allowing gas to accumulate on the hospital grounds for about 25 minutes.

The leak later ignited. The subsequent explosion and fire collapsed most of the maternity hospital on Jan. 29.

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Rios also noted that one of the bolts was the wrong size, and the truck's gas meter had been altered to shortchange customers.

Experts said wire and Teflon tape had been improperly used on parts of the truck's valves.

Authorities are seeking the cancellation of the operating permit for the tank farm where the truck was based. The company that operated the truck could also face fines of up to $2.8 million. More than 70 people were injured in the blast. The dead included a hospital orderly, two nurses and two infants.

Most Mexicans still rely on roof-top propane tanks for cooking and heating water. Nationwide, a fleet of about 10,000 privately-operated tank trucks rumble across Mexico filling stationary tanks through hoses and pipes. Many of those trucks are accused of short-changing customers by illegally altering pipes, valves and flow meters to record more gas than is actually delivered.

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