Three people, including a 14-year-old boy, were killed at two different Jewish community facilities in the Kansas City area of the US state of Missouri yesterday.
Authorities said a man was in custody as police investigated whether the shootings were anti-Semitic.
Police said it was too early to determine a motive, but a leading anti-hate group said the suspect arrested in the shooting was a long-time anti-Semite.
The shootings started around 1pm at the Jewish Community Center of Greater Kansas City in Overland Park, Kansas.
Two men were shot in a car park outside the centre, one dying at the scene and the other at a hospital later, police said.
The shooter then drove 1.6km away to the Village Shalom retirement community that offers skilled nursing services and fatally shot a woman there, Overland Park Police Chief John Douglass.
The two male victims were identified as Reat Griffin Underwood, 14, a high school student, and his grandfather, Dr William Corporon, family member Will Corporon said in a statement.
Both were members of the United Methodist Church of the Resurrection.
The 14-year-old was an Eagle Scout and loved camping and hunting with family, Mr Corporon said.
Dr Corporon had moved to the Kansas City area in 2003 to be closer to his grandchildren.
Two people were shot at, but not hit, the police chief said, adding that it appeared the shooter used a shotgun and possibly other types of guns.
"We know it's a vicious act of violence. Obviously two Jewish facilities, one might make that assumption," Police Chief Douglass told a news conference.
The FBI has been called in to help with the investigation, he said.
The suspect was identified as Frazier Glenn Cross, Jr, 73, by authorities in Kansas, and by the Southern Poverty Law Center as Frazier Glenn Miller, who is the former grand dragon of the Carolina Knights of the Ku Klux Klan.
He was being held at a Johnson County detention centre on suspicion of premeditated murder in the first degree and was scheduled to appear in court this afternoon, according to jail records.
The suspect was taken into custody in the car park of a nearby elementary school, Mr Douglass said.
Mr Douglass said he could not confirm reports from witnesses that the suspect had yelled "Heil Hitler" while in the back of the squad car after being taken into custody.
"The suspect in the back of a car made several statements," Mr Douglass said.
"We are sifting through and vetting those for accuracy, number one, and number two, we are looking at them for their evidentiary value."
US President Barack Obama offered condolences. "While we do not know all of the details ... the initial reports are heartbreaking," he said in a statement.