Police said Thursday a bomb blast damaged a natural gas pipeline in western Canada, and they were investigating if the explosion and an earlier one were linked to a threatening letter.

Sgt. Tim Shields said the blast occurred overnight near the border between Alberta and British Columbia. It left a crater in the ground and damaged — but did not rupture — the pipeline, he said.

Police said the explosion seemed related to the bombing of another EnCana gas pipeline Saturday about 30 miles east of Dawson Creek. That blast left a small crater under the line, but the company said it did not rupture and no gas was released.

There was no claim of responsibility for either explosion, but police believe they may be related to a letter sent to local media outlets that demanded oil and gas projects be shut down.

"We will no longer negotiate with terrorists, which you are as you keep endangering our families with crazy expansion of deadly gas wells in our home lands," said the anonymous letter, parts of which were published by the Dawson Creek Daily News.

The letter did not make specific threats.

Some critics fear the gas, which contains hydrogen sulfide and can be fatal if inhaled, poses a danger to communities near the pipelines.