Billings, MT – A home is destroyed after a gas explosion started a fire at 920 S. 30th St. Sunday, June 20, 2010.

An explosion and fire that severely burned a Billings man Sunday morning was caused by a natural gas leak, authorities said.

The 51-year-old man apparently ignited the gas when he lit a cigarette just before 7 a.m.

Two walls blew off the house at 920 S. 30th St. The occupant was able to get outside on his own but was later transported to a burn center in Salt Lake City with serious injuries.

Authorities did not release his name because they had not been able to contact his relatives.

Billings Fire Marshal Mike Spini said the man was in critical condition.