At least 21 transgender women, most of them women of color, have been reportedly killed in the US this year. That number — 21 — may represent a bigger problem than you think: Since trans women of color make up a fraction of the population, LGBTQ and Black Lives Matter advocates are calling the deaths an "epidemic" of violence.

"What we're seeing here is that not only is there an epidemic of violence against black bodies, but, certainly, within black communities, there is an incredible epidemic of violence against black trans bodies," Alicia Garza, the co-creator of Black Lives Matter, told MSNBC. "And when we talk about black lives mattering, we have to make sure that we're talking about all black lives."

Related 9 questions about gender identity and being transgender you were too embarrassed to ask

But are these murders a growing epidemic? The answer, worryingly, is we don't know, even as November 20 marks Transgender Day of Remembrance. Local, state, and federal governments don't collect much data on trans populations or violence against them, which means the 21 deaths we know about are — at best — a bare minimum, and it's unclear how big of a problem those deaths signify.

"There's very little data on violence against transgender women of color," Chai Jindasurat, the co-director of community organizing and public advocacy at the National Coalition of Anti-Violence Programs (NCAVP), told me. "There's so much data that needs to be collected for us to make the case that we know we can and should be making to policymakers about why this issue is so important."

We don't really know how many transgender people are killed

Even finding out the total population of trans people in the US is tricky. The best numbers come from the Williams Institute, an LGBTQ think tank, which estimated in 2011 that 0.3 percent of the US population is trans. But those numbers are potentially outdated, and they're not broken down by, for example, race and gender identity.

That's important, because if we don't know exactly how many trans people are in the US, we can't figure out the rates of violence they face. And further demographic breakdowns could give more insight into who, exactly, is more likely to face violence — whether, for example, trans women of color are at greater risk.

But we also don't know how many trans people are victimized by violence each year. The biggest issue is that trans people often aren't correctly identified by authorities. If a homicide victim has a penis, police will assume that person is a man. If a victim has a vagina, police will assume that person is a woman. That's it. Police will rarely question if the victim identified differently — at least until family, friends, and media bring it up. (As one example of how bad this can be, the Advocate, an LGBTQ news outlet, reported that one woman, Jasmine Collins, wasn't identified as trans for two months.)

Still, based on the data we do have, LGBTQ advocates say that trans women of color are more likely to face violence. "The bodies of trans women of color are the site of multiple forms of deeply historical oppression," Chase Strangio, a staff attorney for the American Civil Liberties Union's LGBT and AIDS Project, told me earlier this year. "That's a critical part of understanding the violence against trans people."

"For a number of years, a majority of homicide victims have been trans women of color"

The best data we have comes from NCAVP. Their numbers are striking: Among LGBTQ and HIV-affected populations, which is a demographic already believed to face disproportionate levels of violence, trans women of color are much likelier to be victimized — 55 percent of LGBTQ and HIV-affected homicide victims in 2014 were trans women, and 50 percent were trans women of color. Considering trans people make up less than 10 percent of the LGBTQ population, this is a shocking disparity.

"Within [LGBTQ and HIV-affected populations], we see a disproportionate impact of violence against transgender women of color, particularly when it comes to homicide," Jindasurat said. "For a number of years, a majority of homicide victims have been trans women of color."

NCAVP collects this data through self-reports and reports from police, media, and LGBTQ advocacy organizations. But Jindasurat acknowledged this tracking method is problematic, because trans victims and survivors of violence often aren't identified as trans. This is especially an issue for homicides, since homicide victims obviously can't verify their gender identities.

"It gets particularly tricky," Jindasurat said. "Sometimes people may not have been out, they may have been questioning, or they may have been out only to a close group of friends but not their full family."

Still, LGBTQ advocates widely consider NCAVP reports the most comprehensive in the country. But, Jindasurat said, the group's count is very likely the minimum. "We're very realistic with ourselves, and know that it's limited in terms of what we're actually able to document," he said. "There may be much more violence that we're not aware of."

We don't know if violence against trans people is on the rise

As for whether there's a rise in violence against trans people, Jindasurat said it's unclear. While there has been an increase in the reported number of killings, that may be due to elevated awareness of trans issues, making it likelier that advocates, friends, family, and media report these acts of violence with a greater focus on trans issues. So the numbers are very likely getting more accurate, and potentially revealing how tragically common this type of violence is — but since the previous decades' reports likely undercounted the full depths of violence against trans people, it's hard to know whether there is an increase.

"Violence against trans women of color has been a major crisis and an emergency for many years"

"We've known that violence against trans women of color has been a major crisis and an emergency for many years," Jindasurat said. "But now we're seeing more visibility, so more people are aware that it's a crisis."

Jindasurat said he expects this to continue: As media outlets, social media, and LGBTQ advocates pay more attention to trans issues, more of the reports that may have been missed in the past will now end up in NCAVP's reports and the news.

But Jindasurat cautions that's not enough. To really get to the heart of the problem, there will need to be a concerted nationwide effort to force local, state, and federal agencies to collect more data on trans people and violence. And that doesn't just apply to law enforcement agencies focused on violence and crimes. Some trans advocates, like actress Laverne Cox, say that the US Census Bureau should begin tracking how many Americans identify as trans. That way, we can gauge not just how many trans people are victims of violence, but also how many trans people make up the US population. Only then can we start fully tackling this problem.

Figuring out these numbers is crucial to stopping violence against trans people

Violence against trans women of color is clearly a serious problem. But we can never address this issue without comprehensive data that gives us a full indication of how common violence against trans people is, where and when it's taking place, which demographic groups are most affected, and so on.

Better data could, for example, reveal geographic trends in violence. Is this violence more likely to happen in the South, or in one particular state? Is it more likely to happen in states that have poor infrastructure for mental health care, perhaps due to a lack of local, state, or federal funding? How much violence is occurring in particularly violent places in the US, versus safer areas where trans people should be just as safe as anyone else? How much violence is affecting homeless trans women versus trans women with a place to live?

Better data could also show if greater rates of violence against trans women of color are a result of explicit hate crimes, how much are a result of socioeconomic factors that put trans women of color at particular risk for violence, and a mix of issues.

People are, after all, much likelier to be exposed to crime and violence if they're poor, homeless, lack a high school or college education, or can't get a job. And survey data shows that trans people are much likelier to face dire circumstances — in large part because of discrimination. For example, the 2011 National Transgender Discrimination Survey found trans and gender nonconforming people are nearly four times as likely to live in extreme poverty as the general population.

Knowing which major problems trans women of color are facing — discrimination, an array of socioeconomic disparities, something else, or a mix of variables — could help guide which policies would best address violence against trans people.

But until we have better data on the trans population and the violence they face, it will be very difficult to target any policy interventions that could help. And the epidemic will, tragically, be much more likely to continue.