In a gently landscaped subdivision in Loudoun County, dozens of Indian Sikh and Hindu immigrants live comfortably among their white neighbors. The kids play soccer and ride bikes together; the parents share gardening tools and phone numbers for electricians.

So residents of the affluent, ethnically diverse Sterling community were shocked and bewildered when the news broke last month that a family of Indian Sikhs who live nearby — a software consultant, his wife and two children — had found an anonymous, threatening letter in their mailbox. Addressed to “The Turban Family,” the letter suggested that the senders were vigilantes on the prowl for Islamist extremists.

“Our people in the neighborhood have been closely watching your activities and figured out you are a close associate of a secret Taliban movement on the US Soil,” said the typewritten letter. “We ask you to leave the country as soon as possible otherwise one of our people is going to shoot you dead.”

National advocates for Sikh immigrants, known for aggressively defending their members against discrimination, sprang into action. They posted the letter on the Web site of the Sikh American Defense and Education Fund, contacted law enforcement agencies and urged the victims to speak out.

The family was so unnerved by the letter that they asked the media not to reveal their names or faces. But Sikh leaders and residents in Sterling said the death threat was a rare, if chilling, exception in a region where their religious sect — mostly made up of middle-class immigrants from India — has established comfortable suburban roots.

“I can understand that people were upset after 9/11, but most people here are educated and open to us,” said D.J. Singh, 62, an accountant from Ashburn, wearing a business suit and a purple turban. “If people mistake us for Muslims, we need to educate them better about who we are, but we must also respect all religions.”

Last week,Loudoun County Sheriff Mike Chapman visited the Sikh temple in Sterling, also known as a gurdwara, to reassure Sikhs that his force was working to protect them. Located in a nondescript industrial park, the temple’s doors opened to a serene sanctuary. Musicians chanted poems accompanied by tabla drums and harmonium, while women prepared a lunch of rice and lentil stew.

Chapman and 10 armed deputies tied on bright orange head kerchiefs to show their respect. They spoke of the Sikhs as “model citizens,” greeted their leaders familiarly and sat cross-legged for the worship service and lunch. Afterward, Chapman said his office and FBI investigators were working hard to “get to the bottom” of the threat.

The message brought some comfort to the software consultant, who had relocated in 2006 after experiencing similar threats while living in Gaithersburg and Leesburg. Once, he said, someone had chalked “Osama go home” on his driveway. Another time, he’d received a letter titled “Death death death.” Until last month, he thought he had escaped the hostility.

“Everyone here has been so nice and welcoming. I was comfortable before the letter, but now I am concerned,” said the man, who wore a brown turban. He said he had no idea who had sent the warning. “I am trying to maintain the Sikh spirit of always being positive. But I just want my family to be safe,” he said.

Sikhs are far from rare in the Washington region. They began immigrating from India in the 1970s, and their numbers have swelled to several thousand. Initially clustered in Montgomery County, many later shifted to Northern Virginia, where they have opened gurdwaras in Manassas, Fairfax, Sterling, Centreville and Ashburn. Many are professionals and business owners, and some have become active in politics.

Because of their beards and colorful turbans, which are ritually wrapped around uncut hair, Sikh men stand out as different; Hindu immigrants generally shave and go bare-headed. But Sikh leaders said it was violent world events — the 1979 revolution in Iran, the Taliban takeover in Afghanistan and the terrorist attacks of Sep. 11, 2001 — that led them to be viewed with suspicion and confused with Muslims.

“In the 1970s, Americans associated turbans with India,” said Rajwant Singh, a dentist in Rockville and prominent Sikh activist. Gurus and sitar music were fashionable, and Indian faiths were seen as mystical and benign. “Then the media started bombarding everyone with images of Ayatollah Khomeini, the Taliban and Osama [bin Laden],” he said. For the first time, “people started associating turbans with Islam, the Middle East and terrorism.”

After 9/11, there were scattered reports nationwide of harassment or attacks on Indian Sikhs, including the killing of an unarmed man in Phoenix. Tougher airport security rules also led to controversy when airport screeners began attempting to search or remove Sikh turbans, which believers consider a sacred “article of faith.” After negotiations, a compromise was reached on the search rules.

But serious threats have remained extremely rare among Asian Sikh immigrants, who number about 600,000 in the United States. Several dozen Sikhs interviewed last week in Loudoun said they had encountered only occasional, minor problems that seemed to be motivated by prejudice against Muslims or Asians. Some mentioned insults scrawled on their cars, children teased in school or promised job promotions that never came.

One Sikh airport worker said an inebriated passenger had repeatedly called him a “towel-head,” but the traveler later apologized after being arrested and ordered to perform community service at a gurdwara.

The erroneous perception of Sikhs as Islamist extremists is ironic, because Sikhism — a faith that emphasizes self-control and spiritual serenity — was formed 300 years ago to oppose Muslim rule in South Asia. Sikh immigrants have cultivated an image of harmony and healing: The Sterling gurdwara offers classes in yoga and meditation; the Rockville gurdwara has an electronic sign that rotates messages of love. All gurdwaras are open to non-Sikhs, and dozens of area members are white American converts.

Moreover, not all immigrant Sikhs hew to the rigorous demands of their faith. In a fast-paced modern society, where men have less time for grooming rituals, a small minority abandon the long hair and turban to fit in more easily. Some parents cut off their sons’ hair to protect them from bullying; others meet with teachers to explain Sikh culture and make sure they watch for incidents.

“We have been very proactive with our children’s schools, and they have been very receptive,” said one Loudoun parent who spoke on the condition of anonymity because of the letter threat. He said he views Sikhs who cut their sons’ hair as making a tragic concession. “I want my children to feel proud of their heritage,” he said.

In the Sterling community one recent evening, several neighbors waved or stopped to chat after work. Rupinder Sidhi, a Sikh dentist dressed in jeans and sneakers, tossed a soccer ball with her two sons. She said her family puts up colored lights at Christmas, and she described the community as “a lovely, safe place.”

Down the block, Nick Draper, 36, organized his kids for a bike ride before dinner. Last month, when a story about the letter threat appeared in a local newspaper, the white business owner showed it to Sidhi, and the two neighbors discussed it with shared concern and astonishment.

“We are a nice blend,” said Draper, ticking off a list of homeowners that included Sikhs and Hindus, Chinese and Vietnamese, white and black Americans. He also noted that local schools were more than half Asian. “It’s an eclectic mix , but we have a sense of family. Our kids go to each other’s birthday parties,” he added. “Things are changing. For anyone with racial biases, this would be an awkward place to live.”