What constitutes a "detention" under the Fourth Amendment?

This court reviews de novo the district court's denial of a motion to suppress. United States v. Low, 887 F.2d 232, 234 (9th Cir.1989). The district court's factual determinations at a suppression hearing are upheld unless clearly erroneous. Id.

Jones argues that a detention occurred when the agents retained his travel documents. A person is "seized" in violation of the Fourth Amendment if, in view of all surrounding circumstances, a reasonable person would not feel free to leave. United States v. Mendenhall, 446 U.S. 544 (1980); Low, 887 F.2d at 234. "The retention of an airplane ticket beyond the interval required for the appropriate brief scrutiny, may constitute a 'watershed point' in the seizure question." Low, 887 F.2d at 235, (citation omitted); see also United States v. Elsoffer, 671 F.2d 1294, 1297 (11th Cir.1982) (retention of defendant's ticket and identification while interrogating him was a detention).