Hayden Panettiere's Neighbor Says Suspected Dealer Visited Days Before Carolina Trip

Hayden Panettiere may have gotten a visit from a suspected drug dealer just days before leaving for South Carolina ... at least according to her next-door neighbor ... and that's huge, because police think she died from a pill laced with fentanyl.

Hayden's neighbor Mia Terrazas joined "TMZ Live" Wednesday and dropped a bombshell ... saying she suspects a drug dealer showed up at Hayden's West Hollywood apartment on the Tuesday before she died on a Sunday in a Greenville Airbnb.

Mia says the guy couldn't get into the building at first and repeatedly buzzed Hayden's apartment ... before Mia eventually let him in and rode the elevator with him.

That's when she says she noticed he was carrying a small bag with "Brian" written on it. That's interesting, considering Hayden's boyfriend is Brian Hickerson and they traveled to South Carolina together.

Mia says the whole interaction immediately struck her as suspicious ... and based on people she'd seen coming and going from Hayden's place over the years, she suspected the man was delivering drugs.

She stressed she never actually saw what was inside the bag ... but we know the feds are trying to link Hayden's death to a drug dealer in Los Angeles, and the DEA raided her West Hollywood home 9 days after her death.

Mia also says she frequently saw Hayden's boyfriend Brian around the building, despite signs being posted telling residents to report if he was there.

As TMZ previously reported, investigators believe someone may have sold Hayden drugs that contributed to her death, and the DEA is now involved in the case.

Sources have also told us authorities believe Hayden died after taking black-market oxycodone laced with fentanyl ... though official toxicology results are still pending.

Mia says authorities have not yet spoken with her ... despite what she says she witnessed just days before Hayden's final trip.

For the full bombshell interview -- including what else Mia says she saw days before Hayden left town -- watch "TMZ Live."