Minnesota Attorney General Keith Ellison and his office will face some unique challenges in seeking convictions of four former Minneapolis police officers charged in the Memorial Day death of George Floyd. As the attorney general himself noted last week, convicting police officers of murder is no easy task.

Former officer Derek Chauvin is charged with second-degree and third-degree murder and second-degree manslaughter. Three other former officers have each been charged with aiding and abetting second-degree murder and second-degree manslaughter.

Here are some specific challenges prosecutors may face:

First, as noted in "Does Derek Chauvin have a chance at a fair trial?" (June 1), inflammatory statements from elected officials may have created substantial pretrial prejudice. This pretrial publicity almost assures that Chauvin's trial will occur outside Hennepin County, which in turn could decrease minority juror representation.

Second, causation will likely play a key role at Chauvin's trial. In Minnesota homicide cases, the state must prove that a defendant's actions played "a substantial causal factor" in causing the victim's death. All three charged crimes against Chauvin require proof of causation. Competing autopsies from the medical examiner and doctors hired by the Floyd family further complicate this issue.

No doubt Chauvin's defense will highlight the Hennepin County medical examiner's findings of "fentanyl intoxication" and "recent methamphetamine use" to argue that Chauvin's actions did not cause Floyd's death.

Third, the new unintentional second-degree murder charge against Chauvin requires the prosecution to prove that he caused Floyd's death while committing an underlying felony. Here, the underlying felony alleged is third-degree assault, meaning that if Chauvin intended to assault Floyd and committed substantial bodily harm, he may be found guilty of felony murder.

Importantly, the state does not need to show that Chauvin intended to cause Floyd's death. Yet it is important to remember that Minnesota law gives police officers broad discretion to use force when arresting a suspect.

Fourth, the aiding and abetting charges against the other three former officers present a significant obstacle. To secure a conviction for aiding and abetting (also called accomplice liability), prosecutors will have to show both that: 1) These other officers knew Chauvin would commit a crime and 2) that they intended their presence or actions at the scene to further his commission of that crime.

Based on the so far limited publicly available evidence, this standard seems difficult to surpass because we have no direct proof of what the other three officers knew while Chauvin knelt on Floyd's neck for about nine minutes. Instead, we must infer their knowledge and intent.

A key issue could be one officer's observation that Floyd lost a pulse followed by Chauvin's decision to continue applying pressure to Floyd's neck without intervention or objection from the other officers.

Lastly, it is important to highlight that Chauvin is charged both with second-and third-degree murder. Third-degree murder is a unique crime in Minnesota, normally involving situations where a defendant acts with a "depraved mind," a phrase equivalent to recklessness, without regard for a particular person. In fact, it is an open question in Minnesota whether a defendant can have a "depraved mind" and direct his or her actions toward a specific person, as Chauvin undoubtedly did here.

Former officer Mohamed Noor is currently raising this issue before the court of appeals after being convicted of the same charge last year in the shooting death of Justine Damond.

In short, Ellison and his prosecutors face a tough test. These cases will likely meander for many months before trials occur. In the end, a jury or judge will weigh the parties' evidence and arguments and decide whether the state has proved the charged offenses beyond a reasonable doubt.

The public should recognize that despite the seemingly clear video evidence, convictions for any of these former officers will not come easily.

Stephen Grego, of Mendota Heights, is a lawyer who works as a judicial law clerk. The views expressed here are solely his own.