UPDATE 2-Canadian police in Montreal suburb probe ricin letter sent to the White House

(Adds description of apartment building, sources investigating other letters; changes dateline, previous OTTAWA)

LONGUEUIL, Quebec, Sept 21 (Reuters) - Canadian police on Monday were conducting an operation in a Montreal suburb related to the ricin-filled envelope sent to the White House, the Royal Canadian Mounted Police said in a statement on Twitter.

Authorities arrested on Sunday a person suspected of sending the deadly poison by mail, addressed to the White House. A source said the person arrested was a woman with Canadian citizenship.

The special Chemical, Biological, Radiological, Nuclear, Explosives team is leading the operation, the RCMP said, without providing further details.

Police roped off a modern beige and brown apartment building and evacuated its inhabitants.

On Saturday, the RCMP confirmed that the letter had apparently been sent from Canada and said that the FBI had requested assistance.

The envelope was intercepted at a government mail center before it arrived at the White House. U.S. agencies were investigating whether other ricin letters were sent by the same person to other U.S. addressees, two sources told Reuters.

Ricin is found naturally in castor beans but it takes a deliberate act to convert it into a biological weapon. Ricin can cause death within 36 to 72 hours from exposure to an amount as small as a pinhead. No known antidote exists. (Reporting by Christinne Muschi and Steve Scherer, additional reporting by David Ljunggren in Ottawa and Mark Hosenball in Washington, writing by Steve Scherer; Editing by Chris Reese and Jonathan Oatis)