US federal agents believe the slain suspect in a deadly attack on worshippers at a Sikh temple in Wisconsin had ties to white supremacist groups, a senior investigator said.
The gunman, named as former US Army soldier Wade Michael Page, 40, opened fire as several dozen people prepared for Sunday morning services in Oak Creek, said officials and witnesses.
Six people were killed and three critically wounded - including the first police officer on scene - in the attack.
Special Agent Teresa Carlson, head of the FBI's Milwaukee field office, confirmed that Page was the subject of a "domestic terrorism" probe following Sunday's massacre.
"We are looking at ties to white supremacist groups," Ms Carlson told a news conference.
"We did not have an active investigation on him before yesterday. No law enforcement agency had any reason to believe he was plotting anything.
"We are working on it as a possible domestic terrorism case. The definition of domestic terrorism is the use of violence for social or political gain. That's obviously what we are looking at."
Page was a former US Army psychological operations specialist who served between April 1992 and October 1998, ending his career at Fort Bragg, North Carolina, the Pentagon has said.
US military sources told Reuters that Page had been discharged from the Army in 1998 for "patterns of misconduct" and had been cited for being drunk on duty.
The six people killed in the shooting were five men and one woman, ranging from 39 to 84 years old, police said.
Officers called to the Sikh place of worship found four people dead inside the building and two outside, Greenfield Police Chief Bradley Wentlandt said.
One of the first officers to respond to frantic 911 calls from the scene was shot several times as he tended to a wounded victim and was in critical condition along with two other victims at Froedtert Hospital trauma centre.
A second officer then exchanged gunfire with the suspect, who was fatally shot.
The injured officer, a 20-year veteran with tactical experience, was "ambushed" as he arrived at the scene.
Police say he was shot eight to nine times at close range with a handgun.
Oak Creek Police Chief John Edwards said the officer remained in a critical condition, along with two other men wounded in the shootings.
The FBI said there was no reason to believe anyone other than slain gunman was involved in the attack.
Federal officials say the gun used in the shooting was purchased legally.
Oak Creek Mayor Stephen Scaffidi has praised the response of police.
"There is no doubt in my mind that the heroic actions of our police officers prevented an even great tragedy," he told a press conference.
"Sunday was a tragic day for our city - especially given the fact it occurred in a place of worship.
"Our thoughts and prayers go out to the families of the victims, to our wounded officer, all our responders – fire, police, other agencies - and the community, which is still in shock."
US President Barack Obama said he was "deeply saddened" by the killings and promised that his administration would provide "whatever support is necessary" to those investigating the shooting.
Sikhism is a monotheistic faith that was founded in South Asia more than 500 years ago. It has roughly 27 million followers worldwide.
There are roughly 500,000 Sikhs in the US, according to estimates. The majority worldwide live in India.
Sikh rights groups have reported a rise in bias attacks since the September 11, 2001 terrorist attacks.
The Washington-based Sikh Coalition has reported more than 700 incidents in the US since then, which advocates blame on anti-Islamic sentiment.
