In the strongest denunciation to date by a U.S. law enforcement official, Attorney General Eric Holder on Friday labeled the attack on a Sikh temple that killed six worshippers "an act of terrorism, an act of hatred, a hate crime."

Holder spoke at an emotional memorial service for the victims of the attack that emphasized healing and forgiveness instead of retribution for the shooting rampage by an Army veteran who killed himself after being wounded by police gunfire.

Since the September 11, 2001 terrorist attacks, Sikhs in America have been targeted by revenge-seekers who apparently have mistaken them for Muslims, perhaps due to the traditional turbans they wear and their dark skin.

"In the recent past, too many Sikhs have been targeted and victimized simply because of who they are, how they look, and what they believe," Holder said. "That is wrong. It is unacceptable. And it will not be tolerated."

He called for a national discussion on changing laws to prevent future shooting attacks, as well as "how we might change the hearts of those so filled with hate that the despicable act we mourn today could ever have occurred." Holder mentioned no specific laws.

Holder also declared the attack at the Sikh gurdwara, or house of worship, in a Milwaukee suburb to be "an act of terrorism, an act of hatred, a hate crime "that is anathema to the founding principles of our nation and to who we are as an American people."

Earlier, Wisconsin Gov. Scott Walker said the Sikh community lived the words of slain civil rights leader the Rev. Martin Luther King Jr. this week by responding with love to the attack.

Wearing an orange head covering in keeping with Sikh tradition, Walker quoted King's assertion that only love can overcome hate, and said he witnessed that truth in the aftermath of Sunday's attack at a Sikh temple in a Milwaukee suburb.

"This week, our friends and neighbors in the Sikh community have shown us the best way to respond is with love," the Republican governor told the hundreds of mourners who filled the Oak Creek High School gymnasium for the service.

As Friday's service began, three Sikh musicians in dark turbans sat cross-legged on the ground next to a row of six coffins and large, framed photos of the dead.

Mourners slowly walked past, including Sikhs in their traditional turbans and non-Sikhs using scarves and handkerchiefs to cover their heads in keeping with Sikh custom.

"You have taken this life. This is your will," a prayer leader said between hymns sung over somber music. "We accept your will. Please give us strength to bear this loss."

At one point, a group of seven uniformed police officers joined the line to pay their respects, with some embracing family members of the victims.

Overhead, a large video screen displayed projected photos of the dead and wounded in the attack, including police Lt. Brian Murphy, who remains hospitalized from multiple gunshot wounds after being the first responder to the temple on Sunday.

Murphy's family released a statement Friday thanking police and other emergency responders who rushed to the scene. "We will never forget their heroic actions and quick thinking that saved so many lives on that terrible day."

The family also thanked medical staff and the Sikh community for including Murphy in prayers.

Two Sikhs wounded in the attack remained hospitalized, Punjab Singh in critical condition and Santokh Singh upgraded to satisfactory, while another was treated and released earlier this week.

The family of Santokh Singh said "we are profoundly touched by the outpouring of support from people across the world. "

Killed were five men -- Satwant Singh Kaleka, 65; Sita Singh, 41; Ranjit Singh, 49; Prakash Singh, 39; Suveg Singh, 84 -- and one woman, 41-year-old Paramjit Kaur.

At Friday's memorial service, relatives of the slain shared memories of their loved ones and encouraged the Sikh community to move past a crime that still remained beyond full comprehension.