The Drug Enforcement Administration has joined the investigation into the death of the actress Hayden Panettiere, a person familiar with the investigation said on Wednesday.

Ms. Panettiere, who was 36 when she died in South Carolina on Sunday, had frequently discussed her struggles with drugs, alcohol and debilitating chronic pain. But in the months preceding her death, she shared more than she ever had in a memoir, “This Is Me: A Reckoning,” and in several interviews.

“I’ve said it before and I’ll say it again: Addicts aren’t patient,” she wrote in the memoir, which was released in May. “When we’re shaking, nauseated, anxious, lonely or depressed, we need instant relief that sends us into that pain-free place called denial.”

The authorities in Greenville, S.C., have said that when emergency medical workers arrived at the apartment complex where Ms. Panettiere was staying, they found her in a state of cardiac arrest and could not revive her.

A person with knowledge of the situation told The New York Times that an apparent drug overdose led to the cardiac arrest. On an audio recording of the Greenville County Fire and Emergency Management’s online feed that roughly matches the time and location of Ms. Panettiere’s death, a person refers to an “overdose” and says that “CPR is in progress.”

The Greenville Police Department said on Monday that there were no signs of foul play in Ms. Panettiere’s death. It did not immediately respond on Wednesday to a request for comment about the D.E.A.’s involvement in the case.

The D.E.A., a federal agency, enforces the nation’s laws regulating controlled substances. Its work typically includes large-scale investigations into, for example, drug cartels accused of bringing cocaine, fentanyl and methamphetamine into the country. But it does work with local law enforcement. In recent years, it helped the Los Angeles Police Department investigate the drugs involved in the death of the actor Matthew Perry.

The federal agency did not immediately respond to a request for comment.

In a police report released this week, an officer who responded to the scene said that a former boyfriend of Ms. Panettiere’s, Brian Hickerson, was with the actress when she died and that he showed an officer a bag of medications she had been taking. Details about the medications were redacted.

Her hairstylist for the last three years, Erick Orellana, said in an interview that Ms. Panettiere had often complained about physical pain.

“I know that she was struggling with her back, being able to get around,” he said. “It was a back injury that was making it hard for her to walk.”

In her memoir, Ms. Panettiere detailed a long history with drugs that began when she was a child. She wrote that when she was a teenager, a person on her team gave her a “happy pill” at an industry event that gave her “energy.”

“Whatever they were — and I suspect they were some sort of amphetamine — I didn’t care,” she wrote. “At 16, they were the gateway drug that ushered me toward the good of pharmaceuticals and the downfall of addiction.”

Years later, Ms. Panettiere found herself in the TV series “Nashville,” in a role that mirrored her issues with substance abuse and postpartum depression.

“When I went home after acting out what I was really going through, the last thing I wanted to do was properly manage or talk about what I was feeling in a healthy way,” she told The Times in 2023. “So I turned to unhealthy coping mechanisms.”

In 2015, she stepped away from filming the show’s fourth season to enter treatment for depression.

Ms. Panettiere said she would wake up shaking, needing an entire bottle of alcohol to get through the day. At other times, she would use opioids to head off the desire to drink. She re-entered a treatment center for eight months in 2021.

Ms. Panettiere was injured on set several times during her acting career, and she complained about severe back pain in a May interview with Women’s Health.

A child actor in the early 2000s, Ms. Panettiere did some of her own stunt work and told the publication that she was bucked off a zebra while making the movie “Racing Stripes.” She also said she fell many times while learning to figure skate for the movie “Ice Princess.”

Two of her vertebrae were smashed into her neck at one point, she said.

A year ago, she said, she woke up feeling “nothing but pins and needles from my waist down.”

“It was like that for so long,” she said, adding that doctors were unable to give her any answers. She wondered if she would be “paralyzed for the rest of my life.”

Malia Mendez contributed reporting.