A sour gas leak at an EnCana well in northeastern B.C. last month may have been caused by sand that rubbed a hole in a metal pipe, according to the preliminary results of an investigation.

About 15 residents of Tomslake fled their homes after a cloud of toxic hydrogen sulphide gas rolled into the community near Dawson Creek on the morning of Nov. 22.

The B.C. Oil and Gas Commission launched an investigation to determine the cause of the leak and why EnCana crews took so long to respond.

'There might have been some sand going through the line, causing some abrasion.'—B.C. Oil and Gas Commission spokesman Steve Simmons

According to a preliminary statement issued Wednesday, the damaged pipe leaked sour gas for at least an hour before the alarm sounded, and it took crews two hours to arrive and shut it down.

By that time residents had left the area on their own, raising concerns about how the warning system and evacuation plan failed.

"This is very important. One of the things the commission is established for is public safety. When we see something like this that has happened close to the residences, we definitely want to get to the root cause," said commission spokesman Steve Simmons.

"We suspect there might have been some sand going through the line, causing some abrasion," he said, adding that the final report will be issued and presented at a public meeting next month.

People in the area have been on high alert after six bombs went off at EnCana facilities and threatening letters were sent to local newspapers over the past year. No one has been charged for the bombings.

(With files from Betsy Trumpener)