Criminal Procedure
Border Search
The government has authority under the Fourth Amendment to conduct suspicionless searches at the border, including removing the gas tank from a car, based on its interest in protecting the border.
This also allows mail entering the country to be inspected without a warrant, as long as there is reasonable cause to suspect that there is something illegal therein.
Reasonable suspicion is sufficient to detain someone at the border while waiting for a warrant to conduct a search.