Hayden Panettiere BF Brian Hickerson Talking to DEA

Big update in the Hayden Panettiere death investigation ... her boyfriend is talking to the DEA as it looks into the actress' suspected overdose, and TMZ has learned investigators are making progress. 

Sources with direct knowledge of the investigation tell TMZ ... Brian Hickerson -- who was in the Greenville, SC condo when Hayden died -- has spoken with the federal law enforcement agency.

We're also told local and federal investigators believe someone sold Hayden drugs which might have caused her death, and that's the main focus of the investigation ... finding the dealer.

One source tells TMZ the authorities are "zeroing in on a potential drug dealer."

In the meantime, investigators are waiting for toxicology results to come back, which could reveal Hayden's cause of death.

As for who may have sold her drugs ... that's still up for debate. No one has been arrested in connection with her death. Hayden had a documented history of drug and alcohol abuse.

As you know ... Hayden died last month after she was found unresponsive in a short-term rental she was sharing with Brian.

Brian's brother, who found Hayden unresponsive as Brian slept, says they gave her Narcan -- the drug meant to reverse the effects of an opioid overdose -- but to no avail.

Still, the fact that they even had Narcan with them is telling.

Hayden's mother, Lesley Vogel, has accused Brian of being culpable in her daughter's death. He's been kept quiet in the weeks since Hayden died, outside of releasing a short statement through his attorney.