Carrie Fisher went into cardiac arrest on a flight from London to Los Angeles amidst her book tour, a law enforcement source told CBS News.

Todd Fisher said Friday night that his sister is receiving excellent care, but that he could not classify her condition. He had earlier told The Associated Press that she had been stabilized and was out of the emergency room. In a subsequent interview he said many details about her condition or what caused the medical emergency are unknown.

Fisher, 60, experienced medical trouble during a flight from London and was treated by paramedics immediately after the plane landed in Los Angeles, according to reports citing unnamed sources.

Several people who were on the flight tweeted that they saw the veteran “Star Wars” actress receive CPR from passengers who may have been nurses. The plane was then met by paramedics at LAX who provided advanced life support and aggressively treated the patient.

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United Airlines said that medical personnel met United flight 935 from London to Los Angeles upon arrival Friday and after the crew reported that a passenger was unresponsive. Sources have told CBS News that Fisher was that patient.

TMZ first reported that Fisher had gone into cardiac arrest. The Los Angeles Times reported that Fisher was in critical condition.

Actress Anna Akana, who also calls herself a fan of Fisher’s, said she was on the flight and tweeted, “Don’t know how else to process this but Carrie Fisher stopped breathing on the flight home. Hope she’s gonna be OK.” She also added that the flight crew and doctor and nurse passengers jumped into action during the episode. Her boyfriend, comedian Brad Gage, tweeted, “I’m in complete shock. @AnnaAkana and I sat in front of Carrie Fisher on our flight from London and she was just taken off the plane by EMTs.”

Don't know how else to process this but Carrie Fisher stopped breathing on the flight home. Hope she's gonna be OK 😞 — Anna Akana (@AnnaAkana) December 23, 2016

Fisher’s “Star Wars” co-star Mark Hamill tweeted, “as if 2016 couldn’t get any worse... sending all our love to @carrieffisher.”

as if 2016 couldn't get any worse... sending all our love to @carrieffisher — Mark Hamill (@HamillHimself) December 23, 2016

Her half-sister Joely Fisher tweeted: “Use the force babe.”

Use the force babe💋❤️ pic.twitter.com/WVy6vG8ewc — Joely Fisher (@MsJoelyFisher) December 24, 2016

Actors Gwendolyn Christie and William Shatner also tweeted their support for Fisher.

“Sending you the universes most powerful Force,” wrote Christie.

I ask everyone to stop for a moment and send special thoughts to @carrieffisher. — William Shatner (@WilliamShatner) December 23, 2016

@carrieffisher The whole world is sending you so much love! Sending you the universes most powerful Force XXXXX❤❤❤ — Gwendoline Christie (@lovegwendoline) December 23, 2016

Fisher has been open about her past struggles with drug and alcohol abuse, as well as mental illness. In 2013, she was hospitalized due to an incident relating to her bipolar disorder.

In the actress’ latest book is “The Princess Diarist,” but perhaps her most well-known book was 1987’s “Postcards from the Edge,” a semi-autobiographical novel that was adapted into a film of the same name. Her most recent movie was “The Force Awakens,” and she is set to appear in “Star Wars: Episode VIII” in 2017.

Fisher is the daughter of Debbie Reynolds, and her own daughter, Billie Lourd, is an actress on “Scream Queens.”