Officials are investigating a fire at a Shell plant near Edmonton that sparked an evacuation and complaints from neighbours who say they didn't get a warning for hours.

A cloud of sour gas caught fire inside the Scotford heavy oil upgrader in Fort Saskatchewan just before 11 a.m. Thursday, according to Alberta's Energy and Utilities Board.

Shell evacuated the plant and usedthe community update line toplace a warning call to neighbours to stay indoors,company spokesman Randy Provencal said.

However, Maureen Chichak, who lives next to the plant, says she didn't get the warning until 2:30 p.m., despite several calls to the plant.

"I am very upset," she said. "Their emergency response plan has still got holes in it."

Just 10 days ago, theEUB agreed to a major expansion that almost doubles the size of the heavy oil upgrading plant— but only after noting residents' complaints regarding how the company had notified them about past incidents.

Shell says it's regrettable that some neighbours didn't get a call sooner.

Sour gas, also called hydrogen sulphide, did notescape from the facility, said EUB spokesman Darin Barter.

"Obviously there's some public concern out there. We're taking it seriously. We're investigating the incident itself, as well as the response to it," he said.