7:40 PM PT -- Although police say Liam's behavior sounds consistent with that of "cristal" users, they still don't know if that drug was present in the room. We're told they're still waiting for test results on the narcotics and medications they recovered.

Liam Payne was potentially under the influence of a potent drug, known to cause psychotic attacks and hallucinations, when he died ... according to police in Argentina.

Cops in Buenos Aires tell TMZ ... their operating theory is Liam was high on a substance called "cristal," which police say causes users to experience extreme highs and extreme lows, often making them aggressive.

Photos from inside Liam's hotel room show the place trashed ... his TV was smashed and there was drug paraphernalia in the room, which was littered with broken objects.

Cops say drug use might be the reason his room was such a mess ... and it could also say something about his state of mind at the time of his apparent suicide.

Police have said Liam jumped to his death from the balcony of his third-floor suite at the CasaSur Palermo Hotel ... and it sounds like this drug, and its effects, might be to blame for his erratic behavior.

Police say they don't know for sure why Liam jumped ... but one theory is he may have been hallucinating as a result of the illicit substance he was using. The investigation is ongoing.

As we reported ... a couple years ago Liam revealed he had suicidal thoughts back in the day when he was hooked on drugs and touring with One Direction.

But, as we first told you, earlier this year Liam was telling people he was sober.