Los Angeles (CNN) A Southern California doctor convicted of murder in connection with the overdose deaths of three of her patients was sentenced to 30 years to life in prison Friday by a judge in Los Angeles.

Hsiu-Ying "Lisa" Tseng, 46, dressed in a blue Los Angeles County Jail jumpsuit, showed no emotion as the sentence was read aloud by Los Angeles Superior Court Judge George G. Lomeli.

Moments earlier, however, she apologized to the families of her victims, many of whom packed the downtown courtroom.

"I cannot imagine what you have gone through," the bespectacled Tseng said in a soft voice. "I have been -- and will forever be -- praying for all of you."

Tseng is the only doctor in the United States to be convicted of murder for recklessly prescribing drugs , said Los Angeles County Deputy District Attorney John Niedermann, the lead prosecutor on the case.

While the murder conviction against Tseng is apparently without precedent, her prosecution as a doctor-turned-drug dealer is not.

There has been a steady stream of such cases around the country in recent years. Some have targeted allegedly corrupt doctors on the payroll of criminal "pill mill" operators. Others involve self-employed physicians like Tseng accused of abandoning patient care in pursuit of greed.

Medical experts and law enforcement officials blame reckless and criminal prescribing of opiate-based painkillers and other potent narcotics by doctors like Tseng for contributing to the problem.

Mark Nomady, a veteran DEA agent who helped pioneer the prosecution of "doctor cases" in Southern California, said he frequently saw pill abusers whose drugs were supplied by doctors wind up with needles in their arms.

"You can draw a straight line from pharmaceutical opiates to shooting heroin," said Nomady, who retired from the Drug Enforcement Administration in October after 30 years on the job. "At some point they can't afford the pills, the doctor gets popped or whatever, and there they are."

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Prosecutors charged Tseng with second-degree murder for the deaths of Joey Rovero, 21, Steven Ogle, 25, and Vu Nguyen, 28.

Niedermann wrote in court papers that the murder charges were warranted, in part, because Tseng was already on notice about three earlier patient deaths.

Matthew Stavron, Ryan Latham, and Naythan Kenney fatally overdosed in 2007 and 2008 after Tseng prescribed them drugs. The young men each drove long distances to come to Tseng's strip mall clinic for prescriptions. Stavron made a two-hour round trip from his parents' home in San Clemente.

Tseng got a phone call from the coroner's office alerting her to each of the deaths, according to court papers.

Tseng regarded such calls as "just FYI" notices and did not perceive them as a problem, her husband, also a doctor, testified in her defense at trial.

Medical experts and law enforcement officials agree that it's a tiny fraction of doctors who willfully prescribe drugs to patients who don't need them.

But those who do can have a devastating impact on their communities, said Nomady, the retired DEA agent, who worked on more than a dozen such cases in Southern California.

"One doctor can turn a whole town upside down," he said.

Tseng's case is in some ways emblematic of the role so-called dirty doctors play in creating and sustaining addicts, according to law enforcement officials interviewed by CNN.

She catered to young patients who traveled long distances and paid cash for their prescriptions. She wrote them without performing meaningful medical exams and despite there being no medical necessity for the drugs. She ignored pleas from parents and loved ones concerned about the worsening addiction of sons and daughters, husbands and wives, brothers and sisters.

Joseph Rovero

April Rovero wrote to the judge about how she had been expecting her son Joey home from Arizona State University for winter break when she learned of his death on December 18, 2009.

"Instead of welcoming him back from school and enjoying our usual holiday time together, we were struggling through our shock and grief to write an obituary and pick out a casket, clothes and grave site for Joey to be buried in the day after Christmas," Rovero wrote.

She asked Judge Lomeli to impose the 30-years-to-life sentence prosecutors had requested.

Tseng's court file is filled with similarly heartrending letters from the families of other victims and patients.

Defense: Doctor was in over her head

Tseng's lawyer, Tracy Green, defended the doctor as a well-meaning physician who had gotten in over her head in dealing with manipulative, drug-seeking patients.

On the eve of sentencing, Green submitted a stack of letters from Tseng's supporters and one from the doctor herself. Tseng wrote that she felt "shameful and remorseful" for her conduct.

"I want all the patients and their family members to know I have listened carefully to the grief they've shared, their memories, their losses and the impact on their lives," Tseng wrote. "I have heard, I take it to heart and I will never forget."

Tseng wrote that she lacked sufficient training in prescribing addictive narcotics and was in denial about what was going on in her practice.

"I told myself that my patients' conduct was beyond my control," she wrote.

Before sentencing, Green told Lomeli that Tseng bore some -- but not all -- responsibility for the misery that flowed from her prescription pad. She pointed to the patients themselves, the pharmacies that had filled her prescriptions and other doctors who had also prescribed her patients drugs.

Moments before imposing sentence, Lomeli said Tseng ran a reckless "assembly line" style practice that raked in millions of dollars while patients and their families suffered. He said that while she'd accepted some blame, she was still attempting to deflect some on others.

Tseng, who has two children, ages 8 and 11, could spend the rest of her life in prison. She is not eligible for parole until she has served 30 years of her sentence.

Green said Tseng intends to appeal both her conviction and sentence.

Crackdown on 'pill mills'

Federal authorities in recent years have cracked down on so-called "pill mills" from New York to New Orleans. In 2014, federal prosecutors in New York indicted two dozen defendants -- doctors among them -- for flooding city streets with more than 5 million tablets of the heroin-like painkiller oxycodone.

A year later, the Justice Department announced the "largest pharmaceutical-related takedown in the DEA's history," resulting in the arrests of 280 people, including 22 doctors and pharmacists implicated in a scheme to distribute vast quantities of painkillers and other addictive drugs in Arkansas, Alabama, Louisiana and Mississippi.

There have also been cases against self-employed physicians like Tseng.

Last month, DEA agents raided the office of an Atlanta-area psychiatrist who had 12 patients die of prescription drug overdoses. The doctor was charged with prescribing addictive narcotics to patients who didn't need them. The case, and deaths, remain under investigation.

In December, a Santa Barbara, California, physician was sentenced to 27 years in federal prison after being convicted of catering to addicts who paid him cash for prescriptions. Dr. Julio Diaz, called "the Candy Man" by some of his patients , "turned young people into addicts," according to prosecutors. Diaz was seen as such a threat by fellow doctors that they kept a spreadsheet documenting emergency room visits by his patients. Diaz was linked to 20 patient deaths, though he was not charged with the fatalities.

In 2013, another Southern California doctor, Alvin Yee, was sentenced to 11 years in prison for peddling prescriptions in Orange County Starbucks coffee houses. The doctor was caught on surveillance tape conducting cursory exams of "patients," then issuing prescriptions in exchange for wads of cash. Yee was linked to a pair of patient deaths, according to court papers, but was not charged.

Assistant U.S. Attorney Ann Luotto Wolf, who prosecuted both doctors, said she focused on drug dealing charges because they carried sufficiently stiff sentences and were not subject to cause-of-death determinations, which can be very complicated in doctor prosecutions.

Like Nomady, the DEA agent, Wolf said she was struck by the number of patients who turned to heroin once they got hooked on pills.

"These are people who never in a million years thought they'd be injecting themselves," she said.

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Niedermann, the Los Angeles County deputy district attorney, said he knew nothing about prescription drug cases before being assigned one by chance in 2008. In that case, he won a conviction against a Los Angeles doctor who was raking in $1 million a year, mostly in cash, by catering to addicts. He was sentenced to five years in state prison.

After that case, Niedermann took on another. And then another. He has since obtained convictions against seven doctors for prescribing-related offenses.

With awareness of prescription drug abuse increasing exponentially in recent years, Niedermann said, he has fielded calls from police and prosecutors around the country seeking his advice about whether and how to proceed against a doctor.

"There's a lot of interest," he said. "When you hear somebody has stuck their toe in the pool, everybody wants to know what the temperature is."