Some Louisiana residents may think that, in order to make an arrest, a police officer has to be able to articulate a good reason for detaining a person. However, the truth of the matter is that after the fact excuses for why a person was arrested, so long as they are otherwise legally valid, are perfectly legal in Louisiana.
Just like police are legally allowed to lie to a person in an interrogation, police are also allowed to come up with what many would call "pretexts" for arrests. In other words, a policeman on the spot could simply stop and search a person without having a reason in his or her mind to do so. Without more, the officer's arrest and search would be illegal.