By Steve Scherer OTTAWA (Reuters) – Canadian police are investigating a possible risk to national security that forced parliament to lock down for several hours on Saturday after border agents warned of a bomb threat, a government source said on Sunday. The intelligence branch of the border agency on Saturday warned that vehicles near parliament could be laden with explosives, according to the source, who was not authorized to speak on the record. Ottawa police on Saturday said they had located “two vehicles of interest and two persons of interest”, without elaborating, and said there was no t…