Sioux City, Iowa (CNN) Susan Rice lifts her hands to her face. She pushes aside her gray hair and wipes away tears.

She turns to her family, sitting behind her in a federal court, and mouths "I love you." It may be a while before they're all together again.

Rice turns back to face Judge Mark Bennett as he enters. The federal courtroom is silent except for the clang of shackles on Rice's wrists and ankles and her muffled sobs.

Her emotion is perhaps unsurprising. The grandmother of three faces between five and 40 years in prison for conspiracy to distribute 5 or more grams of methamphetamine. That's the size of a packet of sugar.

What is surprising -- almost stunning -- is the emotion coming from the bench. Bennett is about to sentence Rice for a crime that she admits committing, one for which she believes she deserves punishment. It's the severity of the punishment that has her and her family flabbergasted -- and the judge frustrated.

Bennett seems exasperated, exhausted almost, as he explains he must sentence Rice to a full five years -- the mandatory minimum required by law. It is a sentence he deems unjust, too much for a low-level addict, just for being caught with a certain weight of drugs.

Bennett makes sure the record reflects he felt strongly enough to request that Iowa's US Attorney consider waiving the mandatory minimum. He accepts the defense mitigation that Rice had never been in trouble before she was in her 50s, when she began drinking heavily after a bad divorce and was introduced to meth.

Susan Rice was charged with conspiracy to distributute meth.

She met a mid-level dealer who offered her a mattress in his basement and free meth if she would drive him around.

A willing drug mule to feed her addiction? Yes. But not the drug trafficker or conspirator whom the charges and mandatory minimum sentences were designed to target, the judge believed.

His plea fell on deaf ears. He was told there was no option for Rice to be treated as an exception to the law.

"I strongly disagree with that decision," the judge says firmly from the bench.

It is not the first time he has felt this way. Bennett says 80% of the mandatory sentences he hands down are unjust -- but that he is handcuffed by the law, which leaves no room for judicial discretion to consider a sentence based on individual circumstances of the defendant.

I think it's a miscarriage of justice. Federal Judge Mark Bennett, Northern District of Iowa

Too often, Bennett says, low-level nonviolent drug addicts dealing to feed their habit end up being sentenced like drug kingpins.

Bennett says if he had the power, he would jail Rice for perhaps a year, or 18 months. Across the street in a state courthouse, she would have been put on probation, he says.

"I think it's a miscarriage of justice," Bennett says. "But you know people are entitled to their own sense of what justice is."

In the courtroom, the judge lowers his head and his voice.

"With the greatest of reluctance, I sentence you to 60 months," he says.

Judge carries burden of his sentences

U.S. District Court Judge Mark Bennett on the bench in a federal courtroom in Sioux City, Iowa.

Before Susan Rice even began using methamphetamine, doling out these mandatory minimum sentences was already weighing heavily on Bennett's shoulders. As the number of cases he disagreed with grew during his nearly 23 years on the bench, so did his frustration.

He's watched a stream of what he considers low-level offenders going through the courts turn into a flood. It repeated across the country, overwhelming federal prisons to the extent that half of inmates were serving time for drug offenses.

Bennett hoped the tide was turning after members of both parties began pushing for sentencing reform on both state and federal levels, arguing it had been a huge mistake.

Now Jeff Sessions, Donald Trump's attorney general, has instructed that the law governing mandatory minimums be enforced with renewed vigor

"If you are a drug trafficker," Sessions said after issuing his memo to prosecutors, "we will not look the other way. We will not be willfully blind to your misconduct."

Bennett thinks this approach is unjust. "I basically couldn't live with myself if I didn't speak out," he says, standing in the center of his courtroom only hours after sentencing Rice. "I'm compelled to talk about it because I think it's one of the gravest injustices in the history of America."

Year after year, giving out those sentences, is wearing on him.

"The burden of having given so many unjust sentences is a very heavy thing for me to carry around," Bennett says beginning to choke up.

"I do not consider myself soft on crime, but I consider myself opposed to mandatory minimums for low level non-violent drug dealers who are basically addicts," he says.

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The mandatory minimum sentences affecting today's offenders stem from the mid-1980s crack epidemic and a desire to punish drug traffickers. The statutes were amended to include virtually all those involved in a drug enterprise, but a key clause was added: if an offender helped investigate or prosecute others, he or she could get a lower sentence.

President Obama's first attorney general, Eric Holder, issued new guidelines for prosecutors to pick their charges carefully and not slap nonviolent or first-time offenders with the kind of offenses that would trigger mandatory minimums. It was part of the administration's bid to cut the prison population and support treatment, not incarceration, for those who were criminal only because of their addiction.

It's an effective way of protecting the public. Larry Leiser, Pres. of National Association of Asst. US Attorneys

The National Association of Assistant US Attorneys, made up of those who prosecute federal cases, supports Sessions' push to charge the most serious crime that is provable.

"It's an effective way of protecting the public and it has served us well for an awful long time," the group's president Larry Leiser says. "People who were eligible for mandatory minimums are truly people who are involved in significant quantities of these very dangerous substances."

He rejects recent efforts to relax sentencing laws. And he rejects the view the law unfairly catches non-violent addicts who are simply feeding their addiction by selling drugs. And he hails the provision that lets offenders help themselves to lower sentences if they in turn help the authorities take serious criminals off the streets.

She says her sentence did not fit her crime

Mandy Martinson holds a letter she received from President Barack Obama.

But that reasoning has a fatal flaw for Mandy Martinson, who had to live with the consequences of the law.

She was arrested with her drug dealer boyfriend and sentenced for the pound of meth and a firearm found with them. She'd been dating him for only about a month but let him and his drugs move in with her, in return for a steady supply.

She got the mandatory minimum of 15 years in federal prison. He did not.

"He got 12 years because he had more information to share ... when you have more information to share, you have more to bargain with." Martinson says. "I had only known him five weeks, I didn't have any information to bargain with."

Like Rice, Martinson is clear-eyed about what she did. She regrets turning to a drug dealer for comfort, and drugs, in reaction to a previous abusive relationship. She didn't know that dealer was under investigation, and had been for two years.

"I do believe I should have went to prison to serve my time for breaking the law," Martinson says.

But Martinson takes issue with the conspiracy charge that she says held her accountable for everything her then-boyfriend had done before they even met. She was sentenced as if she were a drug dealer -- a sentence that does not fit the crime she admits to committing, she said.

She wasn't sentenced by Bennett, but by another man who also thought her sentence unfair, she said. Judge James Gritzner agreed she was no kingpin, but felt he had no choice but to sentence her using laws intended for one, according to court documents.

By the time of her sentencing, Martinson was clean, had returned to work and was in the throes of fixing her life. Gritzner acknowledged he believed Martinson posed little threat in the future. Given the choice, he would have imprisoned her for 7.5 to 10 years. But he did not have that choice.

Still, Martinson took comfort -- a judge believed in her.

Once in prison she met another. Bennett has been visiting prisoners for years, including some he's sentenced and some he has not, to see how they struggle with the long, mandatory terms, but also how they try to turn their lives around -- often becoming rehabilitated long before they are set to be released, he says.

Listening to Bennett gave Martinson another sign she had not been treated fairly.

"I thought, 'Well, there's two judges that I've heard directly out of their mouths say they're against mandatory minimum sentencing,'" Martinson remembers thinking while hearing Bennett in prison. "I thought, 'why doesn't it change?"

She filed appeal after appeal against the length of her sentence.

And then a third, very important man came out on her side.

In one of his final executive acts, President Obama commuted her sentence. She was released after serving 12 years and three months.

Mandy Martinson is grateful President Barack Obama granted her executive clemency.

Martinson is grateful to be free now but will forever mourn the years lost to a longer sentence -- the prime years of her life when she planned to settle down and have a family. It took away from her being able to care for her mother, Cindy, who got cancer during Martinson's ninth year in prison.

She can't help but think that if the judge could have given her the lesser sentence he thought was proper, she could have cared for her mother in her last few months at their home in Mason City, Iowa.

Instead, in the months before her mother died, Martinson got just four hours with her. She was not allowed to attend the funeral.

Today, she is still grateful to be out, to be sitting in the family home. And she is happy to add her voice to the variety of people making arguments against mandatory minimum sentences and the push to have Congress change the law.

"While this policy may seem on the surface to be tough on crime, there is strong data suggesting that it is neither smart on crime nor fair on justice," argued the attorneys general.

Bennett will continue to fight alongside them for lasting changes.

But he also has a chance to see redemption on a small scale. He now oversees Mandy Martinson's parole and -- if and when she shows she is ready and when she has fully paid her debt to society -- he could end her probation and welcome her back to freedom.