Mr. Paul-Hus, if there is any doubt about their identity, any question about criminality, any issue related to a risk to the public, or any doubt that they might not present themselves in the normal course for subsequent proceedings, they can be, and they are, detained. If officials at the border, which includes the RCMP in the first instance and CBSA and IRCC later on, feel there is any doubt about any of those matters, they are detained to ensure public safety.
On May 10th, 2018. See this statement in context.