More than 400 fatal police killings a year are sanctioned by local, state and federal authorities as justified homicides, but the FBI doesn’t specifically track how many times officers are prosecuted for improperly causing a person’s death.

The FBI said Tuesday that it doesn’t keep statistics on specific prosecutions of law enforcement officers because it doesn’t track crimes by profession, though it does track justified homicide rulings.

In 2012, the last year available for full statistics, law enforcement ruled a total of 410 deaths as justified homicides, the FBI said. The annual number has been steady for much of the past few decades, officials said.

The FBI defines justifiable homicide as “the killing of a felon by a law enforcement officer in the line of duty.”

The key word in the definition is “felon,” implying someone who already has been convicted of a crime. That leaves a lot of areas out of the data, including people who have committed crimes but completed their sentences, and civilians at large.

The murky statistical picture emerges as Attorney General Eric H. Holder Jr. and more than 40 FBI agents are descending on Ferguson, Missouri, in the aftermath of the death of Michael Brown, an unarmed robbery suspect shot six times by a police officer.

Police say Mr. Brown charged officers, but his family has decried the shooting as needless. The killing has unleashed more than a week of racial tensions, protests, riots and looting.

Another fatal police shooting occurred Tuesday in nearby St. Louis.

Unlike Ferguson, most police shootings don’t get the high-profile attention of the U.S. Justice Department. They are adjudicated by state and local officials in a process in which discretion and interpretation are often key to the outcome, legal analysts said.

It’s often difficult to tell the difference between an officer acting properly or improperly, said Jens David Ohlin, a professor at Cornell University Law School.

“To decide between a lawful act of self-defense and a case of police brutality might hinge on something as simple as where were the suspect’s arms and in what direction were they moving,” he said.

Investigations into what exactly happened in a shooting depend a lot on the people involved, said Charles Rose, a professor at Stetson University College of Law.

“The closer you get to the ground where the event occurs, the closer or further you get from the truth based on the nature of the police department, their integrity and the nature of which it trains,” he said.

Training, Mr. Rose said, is a critical part of police action and needs to be investigated more closely.

“Cops by and large are like soldiers: They react in the moment in the way they were trained,” he said.

The post-Sept. 11 era has resulted in new training for police officers and deadly military-grade equipment for local police departments, which could affect the culture of those on the front lines.

“It could be because we’ve started to train them that way because of our shift in training law enforcement due to our concerns about terrorism,” Mr. Rose said.

Whether the justified homicide figure of 400 a year is a complete picture is open to debate.

The vast majority of crime data collected by the FBI and Justice Department are voluntary, meaning other instances of crime could go unreported.

Indeed, the 410 number from 2012 does not include what some call “unjustifiable homicide” — instances in which police, in the course of their duties, kill someone but prosecutors later find no defendable reason.

Much of the judicial debate in Mr. Brown’s death has focused on whether the police were “justified” in their shooting. Was Mr. Brown a dangerous criminal who posed a threat to police and surrounding residents? Or was he an unarmed civilian?

The Justice Department’s annual statistics don’t list crimes committed by law enforcement officers at any level. Nor do they divide crimes by any kind of profession. The types of crimes and severity, along with the race, gender and age of the convicted, are listed, but no indication is given to their jobs.

If the Justice Department starts putting more scrutiny on crimes or questionable activity by police officers, it may take a few years before the information is known.

Most areas of crime reporting and statistics are just now publishing their findings from 2010 and 2011.

The federal government often doesn’t have disciplinary control over the police, Mr. Rose said. That falls to state governments. Instead, federal prosecutors turn to a different course.

“It normally becomes a civil rights action,” he said, pointing to the federal government’s prosecution of the officers involved in the Rodney King case.

“That’s where they prosecute because they don’t often have jurisdiction for the underlying assault or murder,” Mr. Rose said.

In the meantime, riots and peaceful protests continue in Ferguson. The most recent data come from the FBI’s preliminary Uniform Crime Report, covering January 2012 to June 2013.

According to that report, the overall crime rate in Ferguson has dipped over the past 18 months. The only offenses that have ticked upward are property crimes, larceny and motor vehicle theft.

Mr. Holder will visit Ferguson on Wednesday to meet with Justice Department personnel about the investigation into Mr. Brown’s death.

“I realize there is tremendous interest in the facts of the incident that led to Michael Brown’s death, but I ask for the public’s patience as we conduct this investigation,” Mr. Holder said in a statement Monday. “No matter how others pursue their own separate inquiries, the Justice Department is resolved to preserve the integrity of its investigation.”

He said a thorough investigation would aid in “restoring trust between law enforcement and the community, not just in Ferguson, but beyond,” and called for an end to the violence in the town, including “looters and others seeking to inflame tensions.”

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