(CNN) Earlier this month, Prabhjot Singh sat down with his 4-year-old son Hukam and tried for the first time to explain the horrific incident that altered the Manhattan family's life.

"A few years ago, a few men hurt me because of what I looked like, because they thought I was bad," Singh said.

Hukam stared back, confused. "Why?" he asked.

"Their hearts were asleep and they were not thinking about Papaji as a person," Singh said, using a Punjabi term of respect for father.

On the night of September 21, 2013, Singh, a highly accomplished doctor and professor, was walking with a friend on 110th Street near Central Park. Both men are Sikhs and have long beards and wear turbans. Singh heard someone yell: "Terrorist, Osama, get him."

Singh ran but not fast enough. A group of boys and young men on bicycles taunted him using racial slurs. One pulled his beard and then the attackers punched and kicked him repeatedly. He lay on the ground, waiting for them to stop, when passers-by intervened. Singh ended up in the hospital with a broken jaw, dislodged teeth and other gruesome injuries.

But Singh expressed gratitude. He understood it could have been so much worse.

Fifteen years ago Thursday, an Indian Sikh immigrant was gunned down at the gas station he managed in Mesa, Arizona. It was the first revenge killing in the wake of the September 11, 2001, terrorist attacks. The assailant said he wanted to "go out and shoot some towel heads" for the actions of Osama bin Laden.

Hate crimes against Muslims and those perceived as Muslims spiked after 9/11. Sikh men grow long beards and wear turbans as a commitment to their faith, and many Americans mistake them for Muslims.

Sikhs under attack Violent attacks on Sikhs spiked after September 11, 2001. Read about some of the incidents reported as hate crimes.

Reports of incidents in the immediate aftermath of 9/11 and concerns that hate crimes would rise prompted the founding of the Sikh Coalition, which has grown into the largest Sikh advocacy and civil rights organization in America. In the first month after 9/11, the group documented more than 300 cases of violence and discrimination against Sikhs in America.

In the years since, hundreds of hate crimes have been reported, many of them described by police as cases of mistaken identity, like the September 15, 2001, murder in Mesa.

As America marks the 15th anniversary of 9/11 this month, many Sikhs say they feel no safer in this country. A climate of fear has prevailed since the Paris attacks in November; it has surfaced every time shootings and terrorist attacks are blamed on Muslims. Many feel the focus on immigrants in the 2016 presidential election has added to the hatemongering.

"I definitely feel the uptick of more hateful rhetoric in the country," says Singh.

The irony of being attacked was not lost on Singh: He had written about the violence against Sikhs, including a New York Times piece after the 2012 mass shooting at a Milwaukee temple . A year later, he, too, had become a victim, finding himself in the uncomfortable position of having to explain to his young son why he was attacked simply because of the way he looks.

"Whatever I think the environment is around us, I know children absorb all the messages," he says. "How can we prepare them to meet the world that may not be prepared to meet them?

"Why are we being attacked for being Sikh?" he says. "My tradition teaches me to ask what are we doing as a community to have a far more welcoming embrace of people who are different than us."

Photos: American turban For Sikhs, the turban is not about culture, it's an article of faith that is mandatory for men. The turban is also a reason why Sikh men have been targeted and attacked in America, especially after 9/11. Turbans were featured in "The Sikh Project," a 2016 exhibition that celebrated the Sikh American experience. British photographers Amit and Naroop partnered with the Sikh Coalition for the show. This photo is of New York actor and designer Waris Singh Ahluwalia, who was kicked off an Aero Mexico flight in February after refusing to remove his turban at security. Hide Caption 1 of 10 Photos: American turban Harpreet Kaur, a producer at Maryland Public Television, founded Sach Productions, a media organization that creates films focused on minority issues. She was the first Sikh reporter in Washington. Hide Caption 2 of 10 Photos: American turban Harmandeep Singh, a high school senior in New York, arrived from India in 2014. Hide Caption 3 of 10 Photos: American turban Ishprit Kaur's mother is a nurse and inspired her daughter to become one, too. Kaur was also drawn to the medical field because her father is battling Parkinson's disease. Hide Caption 4 of 10 Photos: American turban Japjee Singh, like many Sikh kids, was bullied for years in suburban Atlanta schools. In 2014, the Department of Justice settled a landmark case with the DeKalb County school system that paved the way for better protection from bullying. Hide Caption 5 of 10 Photos: American turban Maj. Kamaljeet Singh Kalsi was born in India but grew up in New Jersey. He was the only Sikh child in his public school and became the first Sikh American to be granted a religious accommodation to serve in the military since a 1980s ban that prevents Sikhs from serving. Kalsi deployed to Afghanistan and now works to end religious discrimination in the military. Hide Caption 6 of 10 Photos: American turban Retired engineer Lathan Dennis-Singh was born in Kingston, Jamaica, where he befriended reggae superstar Bob Marley. He converted to Sikhism 48 years ago at his college in Michigan and has been living in Fairfax, Virginia, for the last 30 years. Hide Caption 7 of 10 Photos: American turban Raghuvinder Singh travels from his home in New Jersey to Oak Creek, Wisconsin, every week to see his father, Punjab Singh, who was shot in the face in the 2012 mass shooting there. Punjab Singh remains paralyzed and communicates through blinking his eyes. Hide Caption 8 of 10 Photos: American turban Sat Hari Singh reversed the New York train he was operating on 9/11 and helped save the lives of many people. He worked with the Sikh Coalition to sue the transportation authority over a policy against turbans and won. Hide Caption 9 of 10 Photos: American turban Musician Sonny Singh is a member of the Brooklyn Bhangra band. In his other life, he's a community organizer who leads workshops on race, religion and social justice. Hide Caption 10 of 10

Sikhism was founded in the 16th century by Guru Nanak in Punjab, an area that is now divided between India and Pakistan. Nanak rejected the rituals involved with other South Asian religions and stressed the importance of good deeds such as serving others and treating all people equally.

The monotheistic religion has more than 25 million followers worldwide and about 500,000 in the United States. Yet a majority of Americans -- 60% -- admitted in a 2015 survey that they knew nothing at all about Sikhs.

Lawyer and activist Valarie Kaur says the threat of violence seems to have become mainstreamed.

Her grandfather settled in California a century ago, and she knows firsthand from her family that discrimination against Sikhs existed long before 2001. But 9/11, she says, was a paradigm shift, a turning point.

She used to talk about living in the "shadow of 9/11." Then the shadow turned out to be long, and what seemed temporary became permanent.

"Bigotry on the fringe has been cemented," says Kaur, whose 2008 documentary, "Divided We Fall," explored Sikhs in the United States and what it meant to be American in a post-9/11 world.

"The threat of hate and racism has become a part of our daily lives," she says.

Despite the current climate, both Singh and Kaur expressed optimism for the future. Both are vocal about their Sikh identities and talk about landmark changes they hope will make things better in America. They point to younger generations of Sikhs who are fiercely proud of their outward identities and to people like Rana Singh Sodhi, who lost two brothers within a matter of months and became a strong voice for his community.

'He was killed simply because of the way he looked'

On Thursday, Singh Sodhi will again gather with family and friends at the corner of 80th Street and University Drive in Mesa, Arizona.

The Chevron station had been the pride of his older brother Balbir, who arrived in the United States in 1988 and worked hard to achieve the American dream.

That dream shattered in 2001, and now a small marble-and-granite memorial bears these words etched in gold lettering: "He was killed simply because of the way he looked."

This is ground zero for the Sodhi family.

Balbir's killer, Frank Roque, told the police: "I'm a patriot and American. I'm American. I'm a damn American."

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Balbir considered himself a patriot, too. An hour before he died, he had driven to a nearby Costco to purchase plants for new landscaping at the gas station. On his way out of the store, he emptied his wallet, donating $74 to the 9/11 victims' fund. Then he called Rana and asked him to bring a few American flags to display.

Balbir was the eldest of eight siblings in a farming family from the Indian state of Punjab. He and his brothers resettled in America and felt indebted to the nation that gave them new opportunities.

But 10 months after Balbir's murder, a second brother, Sukhpal, was also gunned down, while driving his cab in San Francisco. Police said it was not a hate crime, but Rana is certain that both his brothers were killed because of their Sikh identity.

"I understand that everyone has to leave this world but not because we wear a turban, wear a beard," he says. "America is the most diverse country in the world, but people have zero knowledge about who we are."

Rana Singh Sodhi could easily have been consumed by anger or bitterness. Instead he took it upon himself to begin educating people at churches, schools and community events.

He's saddened when he meets Sikh men who shave their beards and abandon their turbans out of fear.

"I don't want to live scared," he says. "This is America. I should be able to live the way I want to live."

Tracking hate, expressing love

Sikh Americans suffered their darkest moment on August 5, 2012, when white supremacist Wade Michael Page went on a shooting rampage at a Sikh gurdwara, or temple, in Oak Creek, Wisconsin. Page killed one woman and five men; all the men were wearing turbans.

At the time, it was the worst hate crime committed in a house of faith since the 1963 church bombing in Birmingham, Alabama. Although the incident made national news, it did not get the same attention as other shooting incidents, says activist Kaur.

"It happened on Sunday. By the following Sunday, we were bumped," Kaur says. "The media did not think the American public had the attention or did not care enough to understand this community. They could imagine a movie theater or an elementary school or a black church. But not a gurdwara."

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Since then, Sikh Americans have made some important gains, Kaur says. That includes an announcement in 2015 that the FBI would begin tracking hate crimes against Sikhs. Sikh Americans had lobbied for years for such documentation, arguing it was a key step in combating such crimes.

The first report will be released later this year.

"It is not going to prevent an Oak Creek massacre, but it was a landmark civil rights victory," Kaur says.

Ultimately, she says, the only way to prevent violence and discrimination is for people to get to know each other. Oneness and love, core foundations of the Sikh faith, are the only way to save the Sikh community, she says.

Sikhism also embraces a concept called seva, which means selfless service. Kaur says Sikhs must go beyond education or lobbying and engage in seva in their neighborhoods to gain full acceptance in this country.

Prabhjot Singh, the Manhattan doctor, agrees.

"I deeply believe in the Sikh spirit of seva," he says. "Working in our communities where we live is one of the more powerful things we can do.

"Shortly after I was attacked, I prepared to shift my work, to work in a community context, and learn how to be more rooted in the work of creating a more loving nation. It's not easy, and I'm no expert. But if anything, being attacked primed me to listen more carefully and feel the consequences of our choices more deeply."

'Wake up their hearts'

When he sat down with his son, Singh told him their lives were forever connected to the lives of his assailants. And that they would spend a long time thinking and talking about it, just like he asks Hukam to think about times he hurts his younger brother.

One man was later arrested in connection with the attack and charged with aggravated harassment and committing a hate crime. No one else has been apprehended.

After the beating, Singh knew it was important for the Sikh community that justice was done, but more important for him, he would like to meet the perpetrators. Perhaps, he says, he is naïve, but he believes that getting to know people who are different is the best way to create change.

Singh would rather his attackers be taught than caught. He wants the streets of Harlem to be safe for his sons. More than anything, he wants that group of young men and boys to stop hating.

Singh posed the question to his son. "What should I do to them if I meet them?"

Hukam smiled. "You have to wake up their hearts."

For Singh, that was the right answer.