EXCLUSIVE: Hayden Panettiere Death Probe Deepens After Bar Brawl

Cops descended on a South Carolina watering hole and slapped handcuffs on Hayden Panettiere's on-off boyfriend Brian Hickerson while also busting his brother, Zach, after a wild barroom brawl – just days before law enforcement sources revealed investigators believe the tragic actress died after swallowing a single oxycodone pill laced with deadly fentanyl, RadarOnline.com can reveal.

Greenville cops raced to Wild Wing Cafe around 9:10 p.m. on Sept. 12 after an assault was reported. Witnesses fingered Panettiere, 37, and Zach Hickerson, 35, as the alleged instigators, while video captured Brian flinging a barstool during the ugly melee.

Panettiere, described by cops as severely intoxicated, was released without charges, while Zach was busted for disorderly conduct and later sprung on a $257.50 recognizance bond.

The barroom bust comes as the investigation into the 36-year-old Heroes beauty's death takes a stunning turn: Multiple law enforcement sources say Panettiere swallowed what she believed was an oxycodone pill the morning she died – but investigators suspect the black-market tablet was secretly laced with fentanyl.

The Drug Enforcement Administration (DEA) and local investigators are now hunting the source of the suspected fatal pill and focusing on a longtime Los Angeles–area dealer who allegedly supplied Panettiere with black-market prescription drugs.

Hickerson, who was inside the Greenville apartment when Panettiere died, has been questioned by federal investigators as part of the widening probe.

Sources now say DEA agents are poring over years of text messages between Panettiere and her alleged dealer – including conversations that continued until the week of her death and reportedly discussed buying pills on the black market. There are "a slew of messages about drugs," one source said.

Panettiere had allegedly obtained several drugs from the dealer and took some while flying from Los Angeles to South Carolina the day before her death.

Neighbors at her West Hollywood condo also reportedly recalled frequent visitors dropping off bags before quickly leaving.

As RadarOnline.com previously reported, Zach said he found Panettiere unresponsive while Brian desperately administered Narcan – the overdose-reversal drug found at the scene – before the brothers performed CPR and summoned help.

Emergency dispatch traffic referenced a possible overdose and cardiac arrest, but paramedics were unable to save the former child star. An autopsy found no signs of trauma that contributed to her death.

As Radar reported, the Greenville County Coroner's Office released the results of its toxicology testing, concluding the actress died from a lethal combination of five different drugs.

In the statement, the medical examiner said the actress had a combination of fentanyl, the anti-anxiety medication alprazolam, a muscle relaxer, and drugs used to treat major depression, bipolar disorder, and schizophrenia in her system.

According to the coroner's report, investigators found blue pressed pills that tested positive for fentanyl, along with a straw on a table and white powder in a sunglasses case inside her suitcase.

While Hickerson's attorney had no comment, Hayden's rep said: "The investigation is still ongoing. Law enforcement has not disclosed who they are targeting, but I am told the case is complex. We hope to have more answers soon."

Panettiere's grieving mom, Lesley Vogel, is determined to get answers as the DEA digs deeper into the actress' final hours and who supplied her drugs, according to an insider.

"The DEA is investigating, it's very serious, and Hayden's mom is clearly on the warpath," a source told RadarOnline.com. "She's not going to rest until she gets justice."