Mexican officials say two worn-out bolts on a gas tanker truck caused a leak that resulted in a hospital explosion that killed five last month.

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

Mexico City chief prosecutor Rodolfo Rios said Monday that two bolts on a flange connected to the truck's pump cracked, allowing a gasket to partially blow out.

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

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.