The West Valley City officer, Jason Vincent, who lives in the neighborhood known as Cranberry Hill, was on his way to work when he noticed a suspicious black SUV, with a man slumped over the wheel.
Vincent called Draper police for backup, and two officers arrived moments later.
Also at the scene was a West Valley City police sergeant, who lives in the neighborhood and was returning home from a graveyard shift when she happened upon the scene.
The officers were about to arrest Nielson on suspicion of a narcotics violation when he broke free and produced a butcher knife while still inside in the SUV.
"When the struggle ensues inside the cab of the [SUV], there's [four] officers on the driver's side attempting to take the gentleman back out of the car," Eining said. "And that's when the knife is produced."
Eining said a Draper officer was between Vincent and Nielson when the knife came out. Vincent spoke aloud that there was a knife, then fired multiple shots, which killed Nielson.
Vincent suffered minor lacerations to his hands, but Eining said at an afternoon news conference that he did not know if Vincent was injured by the knife described as a kitchen or butcher knife with an 8-inch blade or from the altercation in general.
Though the investigation into the shooting is ongoing, Eining's personal opinion is that the officers "did everything they could do in this situation" and acted appropriately.
"We have an officer that recognized a threat and within seconds, he had to identify the threat, and the danger that that person posed to the other officers involved, and had to act on that threat," Eining said. "And again, this is an incident that involved deadly force by a suspect, and was met by deadly force by an officer."
Eining said they found narcotics, but he did not identify what kind.
Police provided no further information on Nielson, whose body remained at the incident scene at least into late Wednesday afternoon. But court records show that when he died, Nielson was facing misdemeanor charges of criminal trespass and assault. In court documents, Sandy police wrote that on Dec. 6, Nielson went into the apartment of his wife's friend, where his wife was sleeping, ripped the sheets off her and pinched her. His wife also had a protective order against him.
In 2004, Nielson faced dozens of charges; in cases filed in Salt Lake, Davis, Utah and Millard counties, he pleaded guilty to at least 20 felony and misdemeanor counts, including burglary, forgery, identity fraud, theft, criminal mischief, disorderly conduct, drug charges and traffic violations, and he agreed to enter a drug treatment center.
In 2010 and 2011, Nielson successfully petitioned the judges to reduce his felony convictions to misdemeanors. That prompted objections from a former friend and co-worker, who wrote to the judges in 2013, claiming that Nielson had threatened him and his family, and would be able to make good on violent threats because he "trains as a professional [mixed martial arts] fighter."
"Jeffrey R. Nielson has a long history of violence, drug abuse and financial related crimes," wrote Josh Lindsay. "While I believe that some people can actually rehabilitate and move forward as productive citizens, I am positive this is not the case with Jeffrey R. Nielson. … Giving [Nielson] a fresh start would be an act of fraud to the general public."
In 2010, Danielle Lindsay, Josh's wife, sought a stalking injunction against Nielson, who she said sent a text to her husband, stating: "I don't want to hurt you or your family, josh, but I will!!!"
Details of the allegations in the 2004 cases against Nielson were not immediately available Wednesday night.
None of the other three police officers fired their weapons during the altercation.
The Draper police officer who was between Vincent and Nielson had a body camera, which was rolling but was knocked away at some point during the altercation. Eining said Wednesday afternoon that police had not yet had a chance to review the footage.
West Valley City Police Chief Lee Russo said Vincent is a 12-year veteran of the force, a K9 officer assigned to the Community Response Unit.
"Officer Vincent is a very competent officer. I would classify him as an excellent officer," Russo said. "I have every confidence, and I am sure that it will bear out that the officer acted appropriately."
Russo commended Vincent for getting involved, even though the situation occurred outside of West Valley City.
"He sees something and he investigates it for the welfare of the individual that he's seeing and for the welfare of his community," Russo said.
No officer wants to start or end their day this way, Russo said of the shooting, adding: "Physically, you could just see [Vincent was] upset with what's happened."
Russo acknowledged that the shooting was "a piece of a broader dialogue going on in this country right now about police tactics and accountability ... The message we are trying to impress on our officers is we are accountable to our public.
"We look to engage in an open, transparent dialogue. We intend to provide as much information as investigation allows."
In December 2013, Vincent shot and wounded another knife-wielding man a shooting that was ruled justified by the Salt Lake County District Attorney's Office.
Vincent shot 24-year-old Oston Shiloh Fairbourn after the man threatened Vincent with a knife and approached to within several feet of the officer on Dec. 15, 2013.
Vincent had seen Fairbourn acting strangely as the man walked in traffic at 3500 S. 3200 West. When Vincent approached Fairbourn, the defendant displayed a large, fixed-blade knife. Vincent shot the man when he refused to drop the knife when ordered.
A 3rd District Court jury later found Fairbourn guilty of first-degree felony attempted murder, and he was sentenced last year to prison for up to life.
The Utah Fraternal Order of Police issued a press release Wednesday saying its 2,300 members were "saddened to hear about yet another potentially deadly assault on Utah police officers ...
"The sobering truth is that if you attempt to inflict death or serious bodily injury on an officer, there is a high chance you will be killed. The men and women of law enforcement took an oath to defend life, including their own. Our society cannot function well if officers' lives are treated as less meaningful than anyone else."
The past four homicides in the state have all been police shootings. Earlier this month, police shot and fatally wounded two other men:
• On Jan. 8, James Dudley Barker, 42, was killed by a Salt Lake City police officer after Barker allegedly hit the officer several times with a snow shovel, breaking bones in the officer's arm and foot. The officer was investigating a call from a neighbor, who reported that Barker who lived a block away was going door-to-door in the Avenues asking if he could shovel snow. Police have said Barker matched the description of a man who had been looking into cars in the area the day before.
• On Jan. 8, Thomas G. Hamby, 49, was shot and wounded by police after he allegedly opened fire on officers responding to his Syracuse home on a domestic disturbance call. Hamby died at a hospital the following weekend, according to police.
And the last homicide of 2014 also was an officer-involved shooting.
On Dec. 28, a Tooele County sheriff's deputy arrived at Nicholas McGehee's Stansbury Park home on a medical call. The deputy shot and killed McGehee, 28, when the man exited the house holding a handgun and pointed it at the officer.
Last year, Utah officers shot and killed 14 people, the highest number in at least five years. Seven other people were shot by law enforcement last year but survived.