MONTREAL -- An envelope addressed to murder suspect Luka Rocco Magnotta caused a complete shutdown Tuesday at one of Canada's biggest postal sorting centres.

The president of the union local said police were called to the Montreal plant after an employee spotted a letter with Magnotta's name as the addressee.

Alain Duguay said police found a white powdery substance inside the envelope.

"That's when they set up a security perimeter and quarantined some people," Duguay said of the police reaction after the discovery.

Police later determined the substance was not dangerous.

The incident forced the centre to close for two hours and several employees were treated for what Duguay described as adverse psychological reactions.

Magnotta is facing numerous charges -- including first-degree murder -- in the death and dismemberment of Montreal student Jun Lin.

The 29-year-old porn actor has also been charged with shipping some of Lin's body parts through the mail.

In May, workers at an Ottawa postal warehouse found a parcel containing a human hand -- addressed to the Liberal party.

Duguay said he wasn't aware of any protocols to intercept letters addressed to Magnotta, but he praised the employee's decision to alert his boss.

"We know that there are investigations on Mr. Magnotta -- I think it was legitimate," he said.

"I don't think one can ever take too many precautions."

Duguay couldn't say whether the envelope was addressed to the Montreal detention centre where Magnotta is locked up.

Montreal police Const. Anie Lemieux would not confirm whether the letter was addressed to Magnotta, but she said the force has launched an investigation.

"It's something that they will look into," Lemieux said of the possible Magnotta connection.

"Our investigators are looking to see where this envelope came from, what the content was exactly, who it was (addressed) to."

A few hours after the envelope was discovered, a Canada Post depot in the Montreal-area community of Ste-Julie was evacuated when staffers there found a suspicious powder inside a bin that came from the Montreal sorting centre.

The Ste-Julie operation was shut down for several hours and officials later determined that the substance was not hazardous, said a spokeswoman for Canada Post.

Anick Losier said that due to the incidents, Canada Post cancelled mail delivery Tuesday in Ste-Julie and the nearby community of St-Amable.

She said she doesn't expect the shutdown of the Montreal distribution centre, which serves most of Eastern Canada, to have a major impact on operations.