It's been nearly two years since the high-profile law enforcement killings of 18-year-old Ferguson, Mo., resident Michael Brown, New York father Eric Garner, and other unarmed black men (or children, in the case of 12-year-old Tamir Rice) sparked months of widespread unrest and calls for police reform across the U.S.

And yet, here we are again as a society in the summer of 2016, still debating and still protesting over the issue of racially motivated brutality.

The back-to-back deaths of Alton Sterling and Philando Castile this week at the hands of police officers in Louisiana and Minnesota respectively set off a new rash of protests that came to a head in Dallas on Thursday when five police officers were killed by a sniper.

Calling for a greater sense of urgency in addressing America's "broader set of racial disparities," U.S. President Barack Obama told reporters on Thursday that  blacks were shot by police at more than twice the rate of whites last year.

The Guardian, however, pegged the 2015 rate of death for young black men, specifically, as five times higher than white men of the same age.

"More black people were killed by U.S. police in 2015 than were lynched in the worst year of Jim Crow," reported Quartz in the wake of Sterling and Castile's deaths, citing different data from either Obama's or the Guardian's. 

"The police are killing people as often as they were before Ferguson," wrote FiveThirtyEight, pointing to not one but six separate data sources for U.S. police killings.

FiveThirtyEight Police killings

Carl Bialik's graphic for FiveThirtyEight shows how much the number of police killings in the U.S can vary based on which tracking project data is sourced from. As it stands, there is no official government database for deaths at the hands of police in America. (Carl Bialik/Fivethirtyeight.com)

While all of these statements may be true, it's hard to say for certain how bad the problem really is without official statistics. 

As it stands, a comprehensive government database tracking how many times American police officers have used deadly force does not exist. Local law enforcement agencies may voluntarily submit the number of people killed each year in the line of duty to the FBI, but it's not mandatory, and it's only done about half the time, according to estimates.  

"You can get online and figure out how many tickets were sold to The Martian," said FBI Director James Comey in October, expressing his frustration. "It's ridiculous — embarrassing and ridiculous — that we can't talk about crime in the same way, especially in the high-stakes incidents when your officers have to use force."

But where policy has failed in this regard, activists, academics and journalists have picked up the slack with extensive research and documentation.

Below are four of the most well-regarded and widely sourced online trackers for civilian deaths at the hands of police in the U.S. today (along with Gun Violence Archive, Killed by Police, and the FBI's own, admittedly incomplete, Supplementary Homicide Reports

'Fatal Force' by the Washington Post

Number of U.S. blacks killed by police, 2015 to present: 381   

The Washington Post's Pulitzer Prize-winning database tracks only deaths in which a U.S. police officer shoots, and kills, a civilian in the line of duty, "the circumstances that most closely parallel the 2014 killing of Michael Brown in Ferguson, Mo., which began the protest movement culminating in Black Lives Matter."

There have been 1,499 such deaths in total across the U.S. since January 2015, according to the Post, which also uses the data to display insights such as "almost all of the people shot and killed by police were men" and "body-worn police cameras were known to be recording in one in eight fatal police shootings."

'The Counted' by the Guardian

Number of U.S. blacks killed by police, 2015 to present:  442

Like the Washington Post, the Guardian has collected data from a shorter span of time than some larger, crowdsourced trackers — only about 18 months.

The database, which counts 1,702 deaths in total, is still one of the most widely regarded. Comey himself gave both the Guardian and the Post an inadvertent compliment last year by telling a private gathering of politicians "it is unacceptable that the Washington Post and the Guardian newspaper from the U.K. are becoming the lead source of information about violent encounters between police and civilians."

Fatal Encounters

Number of U.S. blacks killed by police, January 2000 to present: 2,600-plus

Founded in 2012 after the killing of an unarmed Alabama college student named Gil Collar, Fatal Encounters collects information dating back to 2000 from paid researchers, public record requests, and crowdsourced data.

The project is helmed by D. Brian Burghart, a journalism instructor at the University of Nevada, Reno, with support from data specialists based in Canada, Norway, Australia and the U.S. 

As of June 23, the Fatal Encounters database had a total of 14,042 records for people killed during interactions with U.S. police since 2000, approximately 2,600 of them for blacks and 4,600 for people marked as "race unspecified."

These numbers aren't complete, though. As the group's website states, "we're about 62 per cent of the way to the total of what we think will be about 22,700 total records at the end of 2016."

Mapping Police Violence

Number of U.S. blacks killed by police, 2015 total: 346 

Mapping Police Violence, a research collaborative, bills itself as "the most comprehensive accounting of people killed by police since 2013."

The project's stated goal is to quantify the effects of police violence in communities, and it does this by creating accessible maps, graphs, charts and infographics using data from both external and internal resources: obituaries, social media profiles, criminal record databases, police reports and databases compiled by sites like Fatal Encounters and Killed by Police.

"Law enforcement agencies across the country have failed to provide us with even basic information about the lives they have taken," the website reads. "In a country where at least three people are killed by police every day, we cannot wait for police departments to provide us with these answers."