The troubled star, who boarded a flight from New York to L.A. on Friday, was admitted shortly after touching down in California, PEOPLE has confirmed.
TMZ reports that Bynes has been placed on a 5150 psychiatric hold, where she will be held for 72 hours.
The move comes hours after Bynes lashed out at those she was once closest to: her parents, Lynn and Rick.
On Friday, the troubled former child star, 28, took to her highly charged Twitter account to make a series of alarming accusations, including claims that her father was sexually, verbally and physically abusive when she was a child.
She has since deleted the Tweets about sexual abuse, along with a bizarre retraction of sorts: "My dad never did any of those things The microchip in my brain made me say those things but he's the one that ordered them to microchip me"
In response, Lynn Bynes issued a statement to PEOPLE through her lawyer, Tamar Arminak, refuting the accusations and citing her daughter's "mental state" for charges that "have no basis in reality."
"I am heartbroken today for my husband of 47 years," Lynn said. "Rick has been the best father and husband a family can ask for. He has never abused Amanda or our other children physically or sexually.
"These accusations are absolutely horrible and could not be further from the truth!" she continued. "These allegations stem from Amanda's mental state at the moment. They have no basis in reality. It saddens me beyond belief that my husband's character could be slandered in such a way.”
Through their attorney on Friday, Bynes's siblings, Jillian O'Keefe and Tommy Bynes, told TMZ that they have "the most wonderful parents who have ever lived. ... "We are pained by [Amanda's] behavior but we absolutely give our full unconditional love and support to our wonderful parents."
Long-Standing TensionsWhat no one disputes is that Bynes has had issues with her father for years. A source who has spent time with her in recent weeks says Bynes has talked about her resentment toward her father, holding him responsible for her mounting problems.
"It's scary," the source tells PEOPLE. "She talks about [how] ugly she is, and she sits on her phone 24-7, checking her Twitter feed."
"She is very angry at her father," the source adds.
Until her latest social media outburst, Rick and Lynn remained silent as Bynes seemingly descended into another scary breakdown in the wake of her most recent DUI arrest, on Sept. 28.
Since leaving the home she shared with her parents in Southern California – where she had seemed to be on a healthier path while attending fashion school – and flying to New York City, she has appeared to spiral out of control.
In a series of rambling revelations to reporters and on social media in the past week, she has said she is engaged, needs a "tremendous amount of facial surgery" and "will not be manipulated or brainwashed by anyone."
Past ProblemsThe behavior is chillingly similar to a series of episodes in the summer of 2013, which culminated in Bynes being hospitalized on an involuntary psychiatric 5150 hold. After a period in treatment, Bynes was placed under her mother's care and lived with her parents in the L.A. area.
At the time, her parents believed their daughter was on the road to recovery. "She is doing extremely well," Lynn told PEOPLE in March. "She's making new friends and learning about the fashion industry."
"She's very happy to establish the loving relationship with her family she once shared," Arminak added when she was released to her parents' care.
Back then, Arminak told PEOPLE that Bynes "does not have schizophrenia," as had been widely speculated.
Lynn also issued a statement, saying, "Amanda has no mental illness whatsoever. She has never been diagnosed as schizophrenic or bipolar."
But there is no question that the onetime Nickelodeon star's relationship with her parents, particularly with her father, has long been strained.
While she was hospitalized, Bynes allowed Lynn to visit, but did not want to see her father. "Amanda has consented to seeing her mother, but unfortunately not her dad, with whom she's estranged," said a source at the time.
Early EncouragementRaised in Ventura, California, Bynes was encouraged by Rick, a dentist and sometime standup comedian, to try her hand at improv and acting. "When she was 3, she would put on my clothing and be silly, trying to get attention from her sister and brother," Lynn told PEOPLE in 2004.
Rick encouraged his daughter by writing material for routines about family life, sending her to comedy camp, helping her audition for commercials and serving as her manager.
"When her time onstage was up," Rick told PEOPLE in 2002, "they would physically carry her off. She didn't want to give up the spotlight."
The young performer caught the attention of executives at Nickelodeon as a 10-year-old. That led to a breakout role on the series All That. Bynes became the network's biggest star, landing her own show in 1999.
But by her teens, serious tension developed in the family. Bynes sought to legally emancipate herself from her parents, then withdrew the petition. She also leaned on Nickelodeon producer Dan Schneider and his wife, Lisa Lillien, for support. "She was spending a lot of time with us," said Lillien. "But she never left her [family's] house."
A source who knew the family says Rick was the prime focus of Bynes's anger. "Her parents were very, very strict with Amanda. Her dad called all the shots and was very controlling."
Still, in a 2007 Tonight Show interview with Jay Leno, Bynes took pains to acknowledge her father and mother in the studio audience and credit them with her success.
"My parents actually were very strict, in a good way though," she said. "I was never allowed to go to the mall alone or with friends. But as you can see that's where I got my love of comedy – from my funny dad."
In 2010, however, there were early signs of distress: Bynes announced her retirement from acting on Twitter, saying "being an actress isn't as fun as it may seem." She began turning up in L.A. nightclubs and behaving erratically. That led to a string of car accidents and run-ins with police.
Bynes also became obsessed with her appearance, at one point lashing out at her parents on Twitter: "My dad is as ugly as RuPaul! So thankful I look nothing like you both!"
When confronted with reports about her behavior, Bynes "tells people she doesn't party or drink," a source said at the time. "She doesn't realize she has a problem." At first Rick echoed those sentiments. "My daughter doesn't drink," he told PEOPLE after she was arrested for DUI in April 2012. "She's a good girl."
Up and DownBut as Bynes continued to spiral, her loved ones became worried that she posed a danger to herself and others, and her mother later stated in court papers that she feared Bynes was spending millions she'd earned as a child star on drugs and possibly plastic surgery. After an arson incident outside a home in Thousand Oaks, California, in July 2013, they acted to protect her estate.
"Nobody in the family understood the severity of Amanda's condition until the fire incident," said a source. "Up until then, her family hoped a conservatorship wasn't necessary."
The family's concerns were acknowledged by a California judge, but "the conservatorship was filed on behalf of Lynn Bynes only," said the source. And it was Lynn, not Rick, who addressed their daughter's health in statements to the media.
During a period of recovery at her parents' home, Bynes was seen in the company of both her parents, who seemed optimistic that she was healing. "They hope ... Amanda's situation will get under control," said the source. "They love their daughter very much."
But after a period in fashion school, Bynes's problems have returned, culminating in her return to New York City this month.
So far, Rick has not spoken about his daughter's most recent outburst. But in 1999 he described the importance of family in grounding his talented child. "The reason everybody likes Amanda is who she is," he told the Los Angeles Times, describing a time he turned down a job opportunity in Florida to keep her closer to Hollywood and her loved ones.
"The minute you take her away from here, from her brother and her sister and house," he said, "well, you might not have Amanda anymore."