EXCLUSIVE

2:55 PM PT -- Police now believe it's probably NOT cocaine, but Molly ... a form of ecstasy.



Justin Bieber's BFF Lil Za has just been arrested for COCAINE POSSESSION at the singer's home -- during the early morning raid stemming from the Bieber egg attack on his neighbor.

A rep for the L.A. County Sheriff's Dept. confirmed the arrest for FELONY drug possession ... and said the cocaine was "in plain view" during the raid. Officials say Bieber was "not connected" to the cocaine.

Za's bail has been set $20,000. He is still in custody.

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Lil Za is a rapper who once lived at Bieber's Calabasas home ... and he's been tight with JB for years.

Eleven L.A. County Sheriff's patrol cars descended on Bieber's house this morning to execute a search warrant in connection with Bieber's egg-throwing incident last week outside his neighbor's home ... law enforcement sources tell TMZ.

Sources tell us ... deputies are looking for any evidence that links Bieber to the egg assault.

We're told this is a FELONY search warrant -- meaning Justin is under a microscope for a crime that could land him in prison.

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A Sheriff's official told us they were NOT looking for eggs in Justin's house, but another law enforcement source said cops were searching for anything that was relevant to the egg incident, including eggs and possibly video.

Sheriff's deputies have looked at TMZ's video of the incident and believe Justin has surveillance video that might have captured the egging. We're told deputies dismantled Justin's security system and took it during the search.

By the way ... looking for other eggs may sound silly, but it isn't. There could be some sort of identifier that links what's in Justin's fridge to the broken eggs outside.

Here's the thing ... If in the course of the search cops find something illegal -- like drugs -- it's fair game.

We're told there's a battering ram at the scene.