"...United States through the Federal Bureau of Investigation as well as the local police will conduct a thorough investigation into this crime," Ms Powell told reporters after visiting Gurudwara Bangla Sahib in New Delhi.
She offered her condolences on behalf of the entire US Mission in New Delhi over the incident, which claimed seven lives, terming it a "ghastly act of violence".
"We are deeply saddened by it, particularly that it happened in a house of worship," she said.
The Ambassador was presented a 'Guru Granth Sahib' by the Gurudwara authorities on her visit.
Six people were killed in the attack on the Gurudwara during Sunday morning prayers in Wisconsin by at least one gunman who was also shot dead, Wisconsin police said.
Early in the day, Prime Minister Manmohan Singh expressed shock and sadness over the incident and hoped the authorities there will ensure "conditions" that such violent acts are not repeated.
He welcomed US President Barack Obama's statement on the tragic incident.