Hayden Panettiere's L.A. Home Searched by DEA for Drug Evidence

Hayden Panettiere's Los Angeles home was searched by federal drug agents shortly after her death ... and we're told investigators were looking for evidence that could link the fatal pill to her drug dealer.

Law enforcement sources tell TMZ ... DEA agents executed a search warrant at Hayden's L.A. residence August 25th -- 9 days after her death -- looking for evidence related to black market controlled substances.

We're told investigators were interested in various drugs Hayden may have been taking ... testing them to see if any were laced with fentanyl.

Our sources say agents were also looking for drug paraphernalia, which could provide investigators with a broader picture of Hayden's drug use and potential sources. The search encompassed prescription meds and recreational drugs.

Hayden died August 16 in South Carolina at 36, after being found unresponsive by her boyfriend Brian Hickerson and his brother Zach. Her cause of death has not been officially confirmed -- her autopsy found "no signs of trauma" that would have contributed to her death.

TMZ broke the story ... the DEA became involved in the investigation from the jump, looking for Hayden's "long time drug dealer" in L.A. As we reported, that dealer was providing Hayden with several black market prescription drugs, including oxycodone. Authorities believe she took an oxy pill laced with fentanyl shortly before she died.

Neighbors in Hayden's West Hollywood condo complex tell TMZ ... 3 suspicious people would come to Hayden's door for years, dropping off small bags. The neighbors suspected these were drug dealers.