The Minneapolis police officer who knelt on George Floyd's neck has been arrested. Experts say prosecutors have a long road ahead.

• Local prosecutors have charged the police officer who knelt on George Floyd's neck, and federal authorities have also announced an investigation.
• Now, the question has become how tightly prosecutors can button up their case against the former officer, Derek Chauvin, and whether to charge the other three police officers involved in the incident.
• One legal expert told Insider Chauvin should have been charged earlier — and the civil unrest that has plagued Minneapolis reveals just how deep-seated citizens' lack of trust is in the justice system.
• Other experts told Insider the speed of the arrest isn't nearly as important as ensuring a thorough investigation against Chauvin and the other officers.

The former Minneapolis police officer who knelt on the neck of George Floyd for eight minutes before he died was arrested Friday and charged by Hennepin County prosecutors with third-degree murder and manslaughter.

Within hours, the Justice Department had also announced it was investigating Floyd's death and was looking into charges for possible federal civil rights violations.

Both developments occurred unusually quickly for a case involving an on-duty police officer, legal experts told Insider on Friday. Now, the question has become how tightly prosecutors can button up their case against the officer, Derek Chauvin, and what will become of the three other police officers who were fired over the incident.

The news comes amid growing civil unrest across the country, and particularly in Minneapolis, where protests have turned violent and resulted in riots, looting, and even the burning of a police station. The demonstrations are reminiscent of the 2014 riots in Ferguson, Missouri, after the fatal shooting of Michael Brown, and the 1992 riots in Los Angeles after four officers were acquitted of beating Rodney King.

Mark Osler, a former federal prosecutor and law professor at the University of St. Thomas in Minnesota, told Insider the violence reflects the lack of trust the public has in the government's ability to hold police officers accountable for misconduct. He said that obvious lack of trust was good reason for prosecutors to ensure Chauvin was arrested and charged quickly.

"If we can trust that the government can take care of it, we tend not to riot or try to hurt individuals themselves," Osler said. "People weren't acting very trusting last night. That's for sure."

Osler added that the evidence against Chauvin was so overwhelming that he should have been charged even earlier than Friday, "unless there's something extraordinary that we don't know, that wasn't on the video" that showed Chauvin kneeling on Floyd's neck.

Experts say the speed of the arrest doesn't matter as much as the thoroughness of the investigation

In a press conference on Friday, Hennepin County Attorney Mike Freeman acknowledged the monumental public outrage over the situation but said he would not have laid charges without solid evidence to back them up.

"I am not insensitive to what's happened to the streets. My own home has been picketed regularly," Freeman said. "My job is to [bring charges] only when there's sufficient evidence."

Already Floyd's family has criticized the charges against Chauvin for being too lenient, and for not encompassing the actions of the other three officers involved.

"We want a first-degree murder charge. And we want to see the other officers arrested," the Floyd family's lawyer, Ben Crump, said in a statement. "We call on authorities to revise the charges to reflect the true culpability of this officer. The pain that the black community feels over this murder and what it reflects about the treatment of black people in America is raw and is spilling out onto streets across America."

But other legal experts cautioned that the speed of the arrest won't guarantee the success of the case — only a thorough investigation can do that.

Susan Gaertner, a former Minnesota prosecutor who now works in private practice, told Insider it's responsible for prosecutors to hold off on arrests until thorough investigations are complete — assuming prosecutors have that luxury.

Typically, situations where speedy arrests occur usually involve a fear that the suspect could flee, harm witnesses, tamper with evidence, or commit other crimes unless they're arrested, she said.

"The most important thing is to get it right and not have a major reaction to public pressure to move quickly," Gaertner said.

She added that even though some may think the videos of Chauvin kneeling on Floyd's neck were clear-cut evidence of murder, prosecutors likely have a mountain of other evidence to go through — including body camera footage, surveillance videos from nearby businesses, interviews with witnesses and bystanders, and even an autopsy to formally determine Floyd's cause of death.

Hennepin County prosecutors said it's the fastest they've ever charged a police officer

Ultimately, just four days lapsed between the time when Floyd died and the moment when Chauvin was arrested — that time period pales in comparison to other instances in Minnesota where police officers killed citizens.

After the 2017 fatal shooting of Justine Damond by the police officer Mohamed Noor, Hennepin County prosecutors took eight months before they arrested and charged Noor with murder.

And in 2016, Minnesota prosecutors took four months to investigate and arrest the former St. Anthony police officer Jeronimo Yanez in the fatal shooting of Philando Castile. Yanez was later acquitted of all charges.

Hennepin County prosecutors emphasized on Friday that the charges against Chauvin had been laid with unprecedented speed, which they said was due to the strength of the evidence against him.

"This is by far the fastest we've ever charged a police officer," Freeman told reporters on Friday. "Normally these cases can take nine months to a year."

He added that there were particular challenges involved in bringing police officers to trial, which don't exist when charging civilians.

"We entrust our police officers to use certain amounts of force to do their job, to protect us. They commit a criminal act if they use this force unreasonably," he said. "We have to prove that beyond a reasonable doubt."

Floyd's family has urged that prosecutors charge the other three police officers involved — but one expert warned that's easier said than done

A whole new set of obstacles may well delay any potential charges against the other three officers involved in Floyd's death, according to Bennet Gershman, a former federal prosecutor and law professor at Pace University.

Gershman speculated that perhaps prosecutors will try to charge the men — identified as Tou Thao, Thomas Lane, and Alexander Kueng — as accomplices to the murder Chauvin was charged with. But that effort has its own set of challenges prosecutors will have to contend with.

"Those other officers could be seen as accomplices if they were there with the same intent," Gershman told Insider. "To be an accomplice, you have to share in the intent and the actual conduct of the principal perpetrator."

Prosecutors have mostly declined to discuss the other three officers and any potential charges they face, saying the investigation is ongoing but the priority was "on the most dangerous perpetrator."

On Friday, a new video showed two other officers on the ground with Chauvin and Floyd, both of whom appeared to be pinning Floyd to the ground while Floyd repeatedly begged for air.

Gershman said prosecutors will need to prove that those officers shared in Chauvin's alleged "mental culpability" in killing Floyd — simply "not stopping the person who committed the crime" may not be enough.

"It certainly is much less likely that these other officers would be charged in a particular way as the actual killer. That's clear," Gershman said.

• Read more:
• People keep sharing the video of George Floyd. Some activists and mental health professionals are calling it 'pain porn' and begging them to stop.
• The tragedies of Minneapolis
• Use-of-force experts say George Floyd's arrest was 'horrific' and 'blatantly inconsistent with good police procedure'
• George Floyd's girlfriend says the sight of Minneapolis burning would 'devastate' him, and calls for calm