The prime minister, who is a Sikh himself, also welcomed the statement by US President Barack Obama on the incident.
Speaking to reporters at Parliament House, Manmohan Singh said: "It is a very sad occurrence, the senseless killing of people who gathered at a religions congregation. It happened under tragic circumstances, it makes me enormously sad.
"I hope the American authorities will investigate whoever is behind this dastardly attack on devotees and will ensure that such ghastly events do not take place. The US president has issued a statement, I welcome that. I sincerely hope the authorizes investigate the case and provide relief and succour to affected families. I express my heartfelt sympathies to all the families who lost their kin and those who are injured."
A gunman shot dead six people in a gurdwara in the Milwaukee suburb of Oak Creek, Wisconsin, Sunday.
Separately, in a statement, the prime minister also expressed his shock over the incident.
"I am deeply shocked and saddened to learn of the shooting incident that has resulted in the loss of precious lives and injuries to devotees attending a prayer service at a Gurudwara in Wisconsin, USA.
"That this senseless act of violence should be targeted at a place of religious worship is particularly painful. I send my deepest condolences to the families that have been bereaved in this incident. Our thoughts are with them in this moment of their grief.
"India stands in solidarity with all the peace-loving Americans who have condemned this violence. We welcome the U.S. President's statement on the tragic incident. We hope that the authorities will reach out to the grieving families and ensure conditions that such violent acts are not repeated in the future."