In United States v. Dotson, 799 F.2d 189, 192-93 (5th Cir.1986), the court held that the opinion testimony of three government agents that the defendant and his witnesses were not to be believed under oath was inadmissible because it lacked foundation. 799 F.2d at 193. The only basis offered by the agents was the fact that they had taken part in a criminal investigation of the defendant. The court concluded that because "one had conducted an investigation of the defendant, has known the defendant, or has had minimal contact with defendant's witnesses is not a sufficiently reliable basis under Federal Rule of Evidence Rules 608(a) and 701 for that witness, over objection, to put before the jury the opinion that they are liars." 799 F.2d at 194.