National and state political leaders -- including Gov. Scott Walker -- also offered condolence after the killings, which came two weeks after a massacre at a Colorado movie theater that left 12 dead and dozens more wounded.
Presumptive Republican presidential nominee Mitt Romney called the slaying "a senseless act of violence and a tragedy that should never befall any house of worship."
And from the White House, President Barack Obama said the United States had been "enriched" by Sikhs, "who are a part of our broader American family."
"My administration will provide whatever support is necessary to the officials who are responding to this tragic shooting and moving forward with an investigation," Obama said.
Rajwant Singh, from the Sikh Council on Religion and Education, put the onus on politicians, the media, academics and non-profit leaders to educate Americans about diverse groups and act "to lessen this kind of rage." He called it a tragedy Sikhs, growing up in the United States feel as if they don't belong in this country after incidents such as this.
"Everybody should feel at home," he said. "This nation belongs to everyone."