(CNN) Allegations of excessive police use of force against African-Americans have captured the nation's attention in recent years. But there's another group whose stories you're less likely to hear about.

Native Americans are killed in police encounters at a higher rate than any other racial or ethnic group, according to data from the Centers for Disease Control and Prevention. Yet rarely do these deaths gain the national spotlight.

This lack of attention has prompted some advocates to start social media campaigns reminiscent of Black Lives Matter.

"Native American people are basically invisible to most of the people in the country," said Daniel Sheehan, general counsel for the Lakota People's Law Project

For every 1 million Native Americans, an average of 2.9 of them died annually from 1999 to 2015 as a result of a "legal intervention," according to a CNN review of CDC data broken down by race. The vast majority of these deaths were police shootings. But a few were attributed to other causes, including manhandling. That mortality rate is 12% higher than for African-Americans and three times the rate of whites.

Even though the annual rate of death is higher, the number of Native American deaths is relatively small. An estimated 22 Native Americans and Native Alaskans died at the hands of police in 2016, and another 18 have died so far this year, according to Fatal Encounters , an online database compiled by a former editor at the Reno News & Review in Nevada. It is widely considered one of the most complete sources on deaths resulting from police encounters. CNN excluded deaths caused by car crashes from Fatal Encounters' tally.

This count doesn't include another fatal shooting on Wednesday. A sheriff's deputy shot and killed 14-year-old Jason Pero on the Bad River Reservation in northern Wisconsin. A report by the Wisconsin Department of Justice said that Pero refused to drop a butcher knife and then lunged twice at the deputy. The state Department of Justice, which is continuing to investigate, said the boy himself called 911, giving his own physical description. The Associated Press reported that Pero's family questions the police account and says the boy was home from school sick.

"(There is) no reason you can justify shooting a 14-year-old boy," Pero's mother, Holly Gauthier, told WDIO-TV.

While most fatal use of police force cases that have been investigated are ruled justifiable , some of the deaths caught on video have raised cries of excessive or inappropriate use of force.

Death led to awareness

Paul Castaway's death in the summer of 2015 was one of those controversial shootings that moved his family to fight for wider attention to police violence against Native Americans.

A district attorney's report gave the following account of Castaway's death:

On July 12, 2015, Castaway's mother called 911, breathless. "My son, he pulled a knife on me. He's mentally ill and he's drunk," she said.

Castaway had entered her home without her permission and poked her in the neck with a kitchen knife before running out the back door.

When police arrived, they chased Castaway, who demanded that police kill him and then pressed the knife to his own throat.

Video surveillance footage appears to show Castaway was still holding the knife to his throat with both hands as he walked toward one of the officers.

That officer backed away and fired his gun three times, hitting Castaway twice in the torso. Castaway fell to the ground, and police handcuffed him. He died at the hospital, according to The Denver Post.

Castaway's brother, Gabriel Black Elk, said it took him almost a year to watch the video. "There was a lot of mental anguish we had to go through, me and my mom and my sister."

The Denver district attorney found the shooting justified. The family has filed a lawsuit alleging wrongful use of force and insufficient oversight of officers.

"Police knew they were there to help," Black Elk said. "He wasn't a danger to anybody but himself."

Lynn Eagle Feather holds a bouquet to the sky while clutching a picture in memory of her son Paul Castaway.

Spurred to action by his brother's death, Black Elk, a member of the Lakota Sioux tribe, created a Facebook message group for Native American families who have lost loved ones to police encounters. Slowly, the group expanded to include families of all races.

"A lot of people told me, 'I didn't know this was a problem for Native Americans, too,' " he said.

Deaths are likely underreported

The data available likely do not capture all Native American deaths in police encounters due to people of mixed race and a relatively large homeless population that is "not on the grid," said Matthew Fletcher, director of the Indigenous Law and Policy Center at Michigan State University.

"The numbers might be wildly underreported," he said.

In 1996, American Indian mortality rates were underreported by an estimated 21% because of inconsistencies in identifying Native Americans on death certificates, according to the CDC. The problem has lingered in recent years and is significant enough to make comparisons with other racial groups tricky.

Other media outlets that have kept their own tallies of police-related deaths have reported much higher numbers of deaths than what the CDC publishes. They, too, show high rates of Native American deaths.

The numbers in the Fatal Encounters database, for instance, are more than twice the average number of Native American deaths by legal intervention reported to the CDC.

As Black Elk started to create his Facebook group for grieving families, he said he was just as likely to learn about another Native American death through the grapevine as through local or national media.

Marlee Kanosh says she has sought justice in her brother Corey's death.

Marlee Kanosh, too, lost a brother to police gunfire back in 2012. Corey Kanosh was the passenger in a police chase involving a drunken driver. When the car stopped, he fled police on foot and was shot while resisting arrest. The county attorney concluded that forensic evidence and dispatch logs supported the officer's account of events, but his family complains that he was left overnight at the scene without medical care.

Marlee Kanosh now runs a Facebook page called Native Lives Taken by Police to raise awareness of cases such as her brother's. She said it can be hard to create -- and sustain -- attention for Native Americans' cases, in part because many take place in small communities or more remote areas.

"There are very few people who've heard about a story somewhere out in a small reservation in California, and I see a lot of families who deal with that," she said.

An analysis by Claremont Graduate University researchers recently published in the peer-reviewed journal Race and Justice found that major national or regional newspapers rarely picked up stories about Native American deaths. Even then, it found, the deaths rarely received in-depth coverage. What's more, media don't always correctly identify the deceased as Native American.

A death in Omaha

His brother's death was not the only link Black Elk had to police shootings. His mother's nephew, Benjamin Whiteshield, was killed by police in Oklahoma in 2012. According to the Oklahoma State Bureau of Investigation, his family had brought him to the local police station because he had been "acting delusional." Police said he was holding a wrench and was shot in the mouth after a confrontation with an officer.

Then Black Elk's cousin, Raymond Gassman, was killed in South Dakota less than a year after his brother died. He was shot by tribal police while resisting arrest.

And in June, a member of Black Elk's tribe, Zachary Bearheels, died after a violent encounter with police.

On June 4, Bearheels, 29, was on his way home to Oklahoma when he got kicked off a bus in Omaha, Nebraska. When he failed to make it home, his mother, Renita Chalepah, called police to let them know her son was lost and suffered from bipolar disorder and schizophrenia, the police investigation found.

Omaha police found Bearheels shortly after midnight at a convenience store. The department reported that officers put him on the phone with Chalepah.

"I heard him say 'Mama, mama,'" she later told the Omaha World-Herald . She could tell from his voice that he was off his medications.

According to the police investigation, officers agreed to take Bearheels to the bus station. They handcuffed him and put him in the back of a police cruiser, but he slipped out of the car. That led to a scuffle. Police video shows officers shock Bearheels repeatedly with a Taser, drag him by his belt and ponytail, and punch him in the head.

He was pronounced dead on arrival at the hospital. Two former officers are now facing assault charges. Both have pleaded not guilty.

The police department's investigation found the officers' use of the Taser an "egregious violation" of policy.

"Zachary Bearheels committed no crime," Douglas County Attorney Donald Kleine said at a press conference. "Zachary Bearheels was simply a human being suffering from a severe mental illness that was quite obvious to anyone who came in contact with him. Our laws should protect those who are most vulnerable, particularly those who suffer from mental illness."

Black Elk sees the deaths of his brother and Bearheels as part of a larger problem facing Native Americans.

"It has to do with a mental health crisis and with police killing Native Americans," he said.

Some legal experts with experience working with Native American communities agree that mental health has played a role in the high rate of deaths from police encounters. They say that mental health services for Native Americans are often woefully inadequate.

A 2015 report by the Treatment Advocacy Center , a Virginia-based nonprofit, found that individuals with serious untreated mental illness were 16 times more likely to be killed during an interaction with a police officer and that at least a quarter of fatal encounters involved individuals with serious mental illness.

In February, the US Government Accountability Office placed the federal government's Indian Health Service on its high-risk list . The list highlights agencies and programs vulnerable to fraud or mismanagement or "most in need of transformation."

The report found the agency was ineffective, lacked adequate oversight and put Native Americans' health and safety at risk.

"(Native Americans) do not have anything even approaching reasonable mental health services," Sheehan of the Lakota People's Law Project said of on-reservation health care. "It's staggering."

Addressing the GAO's report, Chris Buchanan, acting director of the Indian Health Service, told the Senate Committee on Indian Affairs in May, "We share the urgency of addressing longstanding systemic problems... We are using the GAO findings and recommendations to inform our strategic and tactical planning efforts."

Some experts also said they believe that pervasive stereotypes about Native Americans may help contribute to highly charged police encounters.

Native Americans are often stereotyped as being violent or addicted to alcohol and other drugs, said Fletcher of Michigan State University. He suspects those perceptions, conscious or not, can sometimes tinge police-civilian interactions.

"If your perception going into a situation is that it's an Indian person and they're completely out of control, I think that changes things on the ground," he said.

Kanosh recalls growing up and seeing "older people get in these scary interactions with police because of alcohol."

"And that would set the bar for future years for cops to always assume that when they were dealing with Native Americans it's going to be because they're drunk and going to get in fights," she said.

A consultant for law enforcement agreed that cultural barriers are a challenge for police in dealing with Native Americans.

"But the flip side of that from an enforcement perspective is police have a job to do, and if it's a polarizing situation leading to a deadly force situation, you don't really have time to consider the cultural aspects of it. You have to take action, whatever that might be," said Rex Scism, president and CEO of Midwest Police Consultants

Scism said he believes those split-second decisions usually fall within the boundaries of the law.

"I'm not going to say the police always get it right; they're human just like everybody else," he said.

Police departments across the country are starting to train officers on how to respond to individuals with mental illness. Many are also incorporating training in cultural awareness and deescalation techniques. Yet changing academy curricula or adopting official training programs is not ubiquitous

Mental illness may have played a role in a police shooting in Winslow, Arizona, a town of about 10,000 that borders the Navajo Nation reservation. The Winslow Police Department faced scrutiny for its role in the 2016 fatal shooting of Loreal Tsingine, a young Navajo woman.

Tsingine was in sweatpants on Easter Sunday last year when officers stopped her in a parking lot on suspicion of shoplifting from a convenience store. Silent body camera footage captures a brief struggle in which Tsingine appears to fall, pulls out a pair of scissors, and then moves away.

She then turns back toward one of the officers, with the scissors in her hand pointed down. The officer shoots her four times: twice in the front and twice in the back as she crumples to the ground, according to the Maricopa County Attorney's Office.

Family members told The Guardian that Tsingine was 5 feet tall and weighed less than 100 pounds. The officer responsible for her death resigned after a meeting with his lieutenant about the internal investigation.

The Department of Justice was called in to examine the police investigation of the case. Prosecutors concluded they could not prove beyond a reasonable doubt that the officer willfully used excessive force, said Devin O'Malley of the DOJ's Office of Public Affairs.

Start of Native Lives Matter

A few of these violent encounters have provoked wrongful death lawsuits or, in Bearheels' case, assault charges . Yet most don't. Kanosh said she and her family tried for years to bring her brother's case to court, but in the end, they were unable to raise the money for an attorney.

Her family is not alone.

"I know plenty of (Native American) families who are even struggling to come up with money for a headstone for their family member," Kanosh said. "They find themselves with not enough money and not enough support, and they give up hope."

But their stories have gained attention on social media. Black Elk and Kanosh both help lead one of the emerging voices for Native American rights, Native Lives Matter. The group began in 2014 and is loosely modeled after Black Lives Matter.

Since its founding, Natives Lives Matter has held rallies and fund-raisers to raise awareness about police violence against Native Americans. Its Facebook page now has more than 160,000 members, and hashtags such as #NativeLivesMatter and #NativeAmericanLivesMatter are slowly gaining currency. A recent event to raise funds for people affected by the Dakota Access Pipeline protests attracted more than 500 people, said Troy Amlee, a core member of the group.

Kanosh has a straightforward goal. "I never want my brother's story to die," she said. "I don't want other people's family members -- brothers, sisters, aunts, fathers, uncles -- I don't want their stories to die either."