Indian Prime Minister Manmohan Singh says he is shocked and saddened by the shooting attack at a Sikh temple in the northern U.S. state of Wisconsin Sunday that left seven people dead, including the gunman.

Mr. Singh, who is a Sikh, called on the U.S. to do everything possible to determine who was behind what he called the “dastardly attack.” India is home to a majority of the world's Sikhs.

Authorities say the gunman shot worshippers in a suburb of Milwaukee, killing six people before he was shot dead by police.

Police describe the unidentified suspect as a white male in his 40s, but have given no details or mentioned a possible motive.

Officials say federal authorities are involved in the investigation because the shooting might be an act of domestic terrorism.

U.S. President Barack Obama said he was “deeply saddened” to learn of the shooting, adding that the country has been “enriched” by the Sikh community.

Sikhism is a monotheistic religion founded more than 500 years ago in the Punjab district of what is now India and Pakistan.

In the U.S., Sikhs, who wear turbans and grow beards, have often been mistaken for Muslims and have been targeted by anti-Islam activists, particularly after the September 11, 2001 attacks.

Authorities say the worldwide population of the Sikhs is 20 million with 400,000 to 1 million Sikhs in the U.S.