The incident began after an employee at the Montreal plant spotted Magnotta's name as the addressee and brought the letter to a manager, said Alain Duguay, the president of the facility's union local.
Police were called to the distribution centre, which handles much of the mail for Eastern Canada. Duguay said an officer unsealed the envelope to find a white powdery substance inside.
"That's when they set up a security perimeter and quarantined some people," he said of the police reaction, which involved about 15 employees and brought operations to a halt for two hours.
Police determined the substance was not dangerous, but four people — two workers and two managers — were treated for what Duguay described as adverse psychological reactions.
Magnotta is facing multiple charges, including first-degree murder, in the death and dismemberment of Montreal student Jun Lin. He 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 Lin's severed hand — addressed to the Liberal party.
The 29-year-old porn actor has pleaded not guilty to all counts.
Canada Post does not have a protocol to intercept letters addressed to Magnotta, so Duguay praised the employee's decision to alert management.
"We know that there are investigations on Mr. Magnotta — I think it was legitimate," said Duguay, who couldn't say whether the envelope was addressed to the Montreal detention centre where Magnotta is locked up pending trial.
"I don't think one can ever take too many precautions."
Neither Canada Post nor Montreal police would confirm whether the letter was addressed to Magnotta.
But Const. Anie Lemieux, a police spokeswoman, 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 letter-carrier depot in the Montreal-area community of Ste-Julie was also evacuated when staffers there found a suspicious powder. The substance was in a mail bin that came from the Montreal sorting centre.
The Ste-Julie warehouse was shut down for several hours and officials later determined that the substance was not hazardous, a spokeswoman for Canada Post said.
Anick Losier did say, however, that five employees in Ste-Julie were taken to hospital as a precaution because they were feeling ill following the incident.
Due to the incident, Canada Post cancelled mail delivery Tuesday in Ste-Julie and the nearby community of St-Amable.
At the Montreal centre, Losier said one employee reported redness on her skin after she came into contact with the substance found in the letter.
She said she doesn't expect the shutdown of the Montreal distribution centre, a plant of nearly one million square feet, to cause a major slowdown for operations.
"Tomorrow, (it) should be back to normal."
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