What is a mistrial?

United States District Judge William Walls declared a mistrial in the Sen. Bob Menendez bribery and corruption trial on Thursday.

The court proceedings and jury deliberations lasted for more than 30 days. 

"I find that you are unable to reach a verdict and that further deliberations would be futile. And there’s no alternative to declaring a mistrial," Walls said to jurors while announcing his decision.

Deliberations had continued into Thursday, days after the jury informed Walls that they were deadlocked, again, and could not reach a verdict.

While Walls requested that the jury continue with deliberations, Menendez’s attorney, Raymond Brown, argued on Monday that Walls should declare an immediate mistrial. Prosecutors then argued that the jury needed more time to deliberate.

In light of the decision, here's what you need to know about mistrials. 

What is a mistrial?

A mistrial, according to the American Bar Association (ABA), is a trial that is not successfully completed -- meaning that it’s “terminated and declared void before the jury returns a verdict or the judge renders his or her decision in a nonjury trial,” according to the ABA.

A mistrial is declared for a variety of reasons, such as the death of a juror or attorney, juror misconduct (if the jury considers evidence that wasn’t presented in the case, for example), a juror discusses the case with the media, or a juror is found to be prejudiced, among other reasons, according to the ABA. A very common reason a mistrial is declared, however, is because the jury is deadlocked and unable to reach a verdict, Adam Winkler, a professor of law at the University of California Los Angeles, told Fox News.

Either side -- the prosecution or the defense -- can make a motion to the judge requesting a mistrial. The judge then either grants or denies that motion. If the motion is denied, the trial continues.

What happens if the motion is granted?  

If a mistrial is declared, one of three things typically happens, according to Winkler: the prosecutor dismisses the charges, a plea bargain or agreement is made, or another criminal trial is scheduled on the same charges.

Going through another trial has advantages and disadvantages for both sides. But a mistrial for the defendant is “always great news,” Winkler said, because the state could choose not to retry the case -- meaning that the charges against the defendant are dismissed. Additionally, the prosecution usually avoids spending the time and money on a second trial if its argument is not strong enough to win, he said.

“Most disputes initially end in settlements because trials are so expensive -- especially in corruption cases,” he said. That's why going to trial more than once is a "daunting task," he added.

If the state does elect to retry the case, however, defense attorneys have specific advantages.

“The defense has already seen the other side’s strategy,” Winkler said, adding that the prosecution is also at a disadvantage because they already disclosed most or all of their evidence during the initial trial.

However, a retrial does give the prosecution an opportunity to come up with a better strategy that could ultimately end in a conviction, Winkler said.

Retrials usually occur in high-profile or political corruption cases, Winkler said -- meaning it’s "very possible" that Menendez could soon see another day in court, Winkler said. 

“It really depends on facts and circumstances of the case, but if there’s political pressure to prosecute then there will most likely be another prosecution.

“It doesn’t hurt that he is a Democrat in a Republican administration -- especially one that has talked about going after its political opponents,” he added.  

What are some famous mistrials?

Bill Cosby

Actor and comedian Bill Cosby’s sexual assault trial ended in a mistrial on June 17. The jury deliberated for more than 52 hours over six days. Prosecutors will retry Cosby, and the new trial is expected to begin in 2018. 

Phil Spector

Record producer and musician Phil Spector was accused of murdering Lana Clarkson, an actress, in 2007. The first trial resulted in a mistrial after jurors deliberated for about 44 hours over 12 days. However, Spector was retried and found guilty of murder in the second degree in 2008.

Shannon Kepler

Tulsa, Okla. police officer Shannon Kepler was accused of killing his daughter's black boyfriend in 2014 while he was off-duty. After three mistrials, Kepler was convicted of first-degree manslaughter during his fourth murder trial in October. He is slated to be sentenced later in November.

Bandidos biker gang

In 2015, the Bandidos biker gang encountered rival biker club, the Cossacks, at a Twin Peaks restaurant in Waco, Texas. A brawl and gunfight was sparked and nine bikers were left dead. Christopher "Jake" Carrizal, the president of the Bandidos' Dallas chapter, testified in court that the Cossacks started the brawl. On Nov. 10, the judge in the case declared a mistrial after jurors deliberated for 14 hours. A retrial for Cossacks was not immediately scheduled. 

Michael Slager

Walter Scott died in 2015 after a white police officer named Michael Slager shot him. A bystander captured the incident on video. Scott was unarmed. After the jury in the case could not reach a unanimous vote, the judge declared a mistrial. But in May, Slager pleaded guilty as a result of a plea agreement with prosecutors. As a result, the state dropped its murder charges against him.