Hayden Panettiere and her drug dealer engaged in numerous text conversations for years -- right up until the week she died -- and the DEA is now laser-focused on the thread of incriminating evidence ... sources tell TMZ.

Sources involved in the investigation with direct knowledge tell us ... Hayden and her dealer were very specific about purchasing black market pills. As one source put it, "There are a slew of messages about drugs." We're told the text messages are now a centerpiece of the investigation.

TMZ broke the story ... the DEA believes Hayden died after taking an oxy pill laced with fentanyl. Our sources say Hayden took various pills with her on the flight from L.A. to South Carolina, the night before she died. Her boyfriend, Brian Hickerson, gave police a bag of Hayden's meds, which could reveal other pills laced with the deadly drug.

And as we reported, Hayden's West Hollywood neighbors say they saw 3 suspicious people coming to Hayden's unit for years. They would come individually, often with a small bag. They saw Hayden open the door, take the bag and the person would leave.

Neighbors suspected the 3 people -- two women and a man -- were drug dealers. The man showed up with a bag the week Hayden died.

As for the text messages, there have been other cases where such messages helped authorities nab the drug dealer -- notably in the Mac Miller case, where the dealer went to prison.

The toxicology results in Hayden's case will not be available for at least a few weeks.