More than half of the 1,166 police killings that occurred in 2015 were not officially documented, according to a new study.

Harvard researchers compared death certificates logged by the Centers for Disease Control and Prevention (CDC) to news reports of people killed by police recorded in the Guardian newspaper's database, 'The Counted.'

The media outlet documented 93 percent of these deaths. The CDC only reported 45 percent.

The number of unreported deaths at the hands of police varied widely from state to state, but the deaths of people under 18, black people, people living in low-income areas and people killed by something other than a gun were most likely to go unreported, across the board.

The Harvard study examined what percentage of death certificates failed to disclose the involvement of police in each state in 2015. The lightest blue states failed to report between zero and 20 percent of police-related deaths. The darkest blue states missed 80-100 percent

The killing of civilians in the US - particularly people of color such as Michael Brown and Eric Garner - have been the subject of protests and headlines in recent years.

This Boston, Massachusetts study's findings suggest that the media has kept better track than US governmental bodies, like the CDC and the Department of Justice.

'There is no federal database that accurately collects data on all people killed by police in the US,' says Harvard study co-author, Justin Feldman. Both the DOJ and the CDC have databases, but this study proves that these are far from accurate.

'When a coroner or medical examiner writes up a death certificate, they will not write up that it's a police killing,' says Feldman.

Under-reporting of killings by police is a problem with a simple direct cause, but its belied by complicated legal inconsistencies.

Each state and even county has different regulations governing death certificates. Beyond that, coroners and medical examiners 'actually have a lot of professional autonomy,' says Feldman.

More than half of 2015 deaths caused by police were not officially documented in death certificates, a new Harvard study found

The study found that every state missed at least some of the deaths. But some were far worse than others.

While Oregon reported the vast majority of its 17 police-related deaths, 'Oklahoma was the worst offender,' says Feldman. There, 36 people were killed by police, and not a single one of their death certificates disclosed that information.

In most counties, coroners are elected by the public, while medical examiners are civil servants employed by the county.

In Oregon, an autopsy must be performed by a medical examiner if the cause of death is anything but natural, including homicides of any kind.

A physician there could sign a death certificate, but 'whenever you have a police shooting, they would immediately call one of the medical examiners,' says Oregon medical examiner Dr Karen Gunson.

The manner of death on such a certificate would be 'homicide' and a medical examiner would complete an injury information form, which would disclose that the fatal injury was inflicted by police.

'Somewhere on that death certificate, it's going to say 'shot by police,' says Dr Gunson.

Death certificates can be signed by either medical examiners or coroners, but these physicians are governed by their individual counties and states, which may contribute to inconsistent practices in reporting that police were involved in deaths

There are six medical examiners across three offices in Oregon. This may be part of the reason that the state's reporting practices are quite consistent.

By contrast, Oklahoma has one chief medical examiner that oversees all of the examinations in the state, says Feldman.

'The implication is that we need to do better. The CDC and other federal agencies need to do a better job counting these deaths,' he says, adding that the CDC, as an important database of vital statistics has a major interest in providing accurate data.

But it's not just any data that is inaccurate. Another study examined death certificates in civilian homicides. These reports were 'highly accurate,' Feldman says.

Beyond inconsistencies in laws and regulations governing death certificates, it's unclear exactly why medical examiners and coroners would not report that police were involved in killings.

'This study didn't look at police influence; that's something that, hopefully, more journalists will look into,' Feldman says.

In fact, he suggested that media reports even be 'incorporated' into databases tracking deaths, and that medical examiners and coroners need better training.