(Reuters) - A utility worker who punctured a natural gas line caused an explosion in a Massachusetts city last week that injured 18 people, a city hall spokesman said on Sunday.

Investigators have found that the Columbia Gas of Massachusetts worker breached the high-pressure line with a probe as he checked for the source of a gas smell on Friday at a downtown Springfield strip club, said Tom Walsh, a spokesman for Mayor Domenic Sarno.

The area rapidly filled with gas fumes and was evacuated. The source of the gas odor and what sparked the explosion itself were unknown, he said.

More than 40 buildings were damaged in the blast. Eighteen people were hurt, none seriously.

Walsh spoke after a news conference on the source of the blast that included Sarno, State Fire Marshal Stephen Coan, the Springfield bomb squad and the Department of Public Utilities.

Columbia Gas of Massachusetts, a unit of NiSource Inc, will open a claims office in Springfield on Monday, Walsh said.

A Columbia Gas spokesman was not immediately available to comment.

(Reporting by Ian Simpson; editing by Philip Barbara)