Cops in Argentina are digging deep to get answers in Liam Payne‘s death, and TMZ has learned they’re questioning all the employees at the hotel where he fatally fell from his third-story balcony.

Law enforcement sources in Buenos Aires tell TMZ … cops are focusing on one hotel employee in particular who they suspect might have supplied the late singer with anti-anxiety meds reportedly found in Liam’s room at the CasaSur Palermo Hotel.

We’re told police are investigating to determine whether the employee supplied Liam with any of the drugs found in his system — including cocaine, crack, ketamine, ecstasy and a form of crystal meth called “cristal” — according to initial autopsy results.

We’re told no final conclusions have been made, and the investigation remains ongoing.

As we reported, preliminary autopsy results revealed Liam had multiple substances in his system when he died last week — including a mix of drugs known as “pink cocaine.”