Liam Payne Death Investigation, Focus on Drugs and Soap Box Found in Room

One small item found in Liam Payne's hotel room is getting a lot of attention from cops investigating his death, because it might unlock the mystery of who helped him score drugs ... TMZ has learned.

Law enforcement sources in Buenos Aires tell us ... authorities are now investigating the possibility a hotel employee delivered drugs to Liam, which has them focusing on a Dove soap box found in the room, and seen very clearly in a photograph taken after Liam's deadly fall.

The box was on the same table where cops also found drugs and paraphernalia -- tinfoil used to ingest drugs, as well as white powder.

On the right side there's the empty soap box. It seems misplaced in the gritty scene, but we're told police have a theory about it.

We're told the operating theory is the soap box might have been used to secretly hand-off the drugs to Liam.

We broke the story ... cops have been looking into hotel employees for several days now -- with sources saying they've got one particular worker in their crosshairs for allegedly giving Liam the anti-anxiety medication they found.

Cops were also investigating whether that employee gave LP the cocaine, crack, ketamine, ecstasy and a form of crystal meth called "cristal" the initial autopsy found in his system.

Late last night, cops raided the Buenos Aires hotel where Liam died ... reviewing electronic and paper records -- all to figure out who sold the former One Direction star the drugs.