Terrified worshippers hid in cupboards and others desperately texted friends outside for help as a man gunned down six people in a deadly rampage at a Sikh temple.
The gunman was later killed outside the Wisconsin temple in a shoot-out with police.
Police in Oak Creek, a suburb of Milwaukee, called Sunday's attack an act of domestic terrorism, but did not provide any details about the gunman or suggest a possible motive. Police Chief John Edwards did not say whether the attacker specifically targeted the Sikh community.
During a chaotic few hours after the first shots were fired, police in tactical gear armed with assault rifles surrounded the Sikh Temple of Wisconsin with armoured vehicles and ambulances. At one point witnesses feared several gunmen were holding women and children hostage inside.
One of the first officers to respond to frantic emergency calls seeking help was shot several times as he tended to a wounded victim and was in a critical condition, along with two other victims, authorities said.
Mr Edwards said the FBI would lead the investigation because the shootings were being treated as domestic terrorism, or an attack that originated inside the US. But it appeared the investigation had moved beyond the temple, as police and federal agents swarmed a neighbourhood in nearby Cudahy, evacuating several homes and sealing off four blocks around a house. FBI agents were on the scene with an armoured truck and other vehicles. Milwaukee County sheriff's spokeswoman Fran McLaughlin said the department's bomb squad also was on the scene, though she had no details about why the unit had been called.
Mr Edwards said the gunman "ambushed" one of the first officers to arrive at the temple as the officer, a 20-year veteran with tactical experience, tended to a victim outside. A second officer then exchanged gunfire with the suspect, who was fatally wounded.
Tactical units went through the temple and found four people dead inside and two outside, in addition to the gunman. The three wounded were being treated at an area trauma centre. Greenfield police chief Bradley Wentlandt, who assisted the investigation, said the police officer had surgery and was expected to survive.
Sikhism is a monotheistic faith founded more than 500 years ago in south Asia, with about 27 million followers worldwide. Observant Sikhs do not cut their hair; male followers often cover their heads with turbans - which are considered sacred - and refrain from shaving their beards. There are about 500,000 Sikhs in the US, according to estimates. The majority worldwide live in India.
Sikh groups have reported a rise in hate attacks since the September 11 2001 terrorist atrocity. The Washington-based Sikh Coalition has reported more than 700 incidents in the US since 9/11, which advocates blame on anti-Islamic sentiment. Sikhs do not practise the same religion as Muslims, but their long beards and turbans often cause them to be mistaken for Muslims, advocates say.