Police and the FBI are trying to determine whether the killing of three Muslims during a neighbourhood parking and noise dispute was a hate crime.
Craig Hicks (aged 46), who describes himself as a "gun toting" atheist, appeared in court charged with the first-degree murder of Deah Barakat (aged 23), his wife Yusor Mohammad, 21, and her sister Razan Mohammad Abu-Salha (aged 19), in Chapel Hill, North Carolina, on Tuesday.
People have said Hicks always seemed angry and frequently confronted his neighbours.
His ex-wife said he was obsessed with the shooting-rampage movie Falling Down and showed "no compassion at all" for other people.
His current wife, Karen Hicks, said he "champions the rights of others" and the killings "had nothing do with religion or the victims' faith".
The killings were "related to long-standing parking disputes my husband had with various neighbours regardless of their race, religion or creed," she said.
She later issued another statement, saying she was divorcing Hicks.
Officers were summoned to the condominium complex by a neighbour who reported five to 10 shots and the sound of people screaming.
The women's father, Mohammad Abu-Salha, said police told him each victim was shot in the head inside the couple's apartment, and that he was convinced it was a hate crime.
"The media here bombards the American citizen with Islamic, Islamic, Islamic terrorism and makes people here scared of us and hate us and want us out. So if somebody has any conflict with you and they already hate you, you get a bullet in the head," said Mr Abu-Salha, a psychiatrist.
The killings are fuelling outrage among people who blame anti-Muslim rhetoric for hate crimes.
A Muslim advocacy organisation pressed authorities to investigate possible religious bias and many posted social media updates with the hashtags MuslimLivesMatter and CallItTerrorism.
About 2,000 people attended a candlelight vigil for the victims in the heart of the University of North Carolina's campus last night.
"We understand the concerns about the possibility that this was hate-motivated, and we will exhaust every lead to determine if that is the case," Chapel Hill police chief Chris Blue said.
Chapel Hill police asked the FBI for help in the probe and Ripley Rand, the US attorney for the Middle District of North Carolina, said his office was monitoring the investigation. But he added that the crime "appears at this point to have been an isolated incident".
Mr Barakat and Ms Mohammad were newlyweds who helped the homeless and raised funds to help Syrian refugees in Turkey.
They met while running the Muslim Student Association at North Carolina State before he began pursuing an advanced degree in dentistry at the University of North Carolina at Chapel Hill.
Ms Mohammad planned to join her husband at dentistry school in the autumn.
Ms Abu-Salha was visiting them from Raleigh, where she was studying design at North Carolina State University.
Imad Ahmad, who lived in the condominium where his friends were killed until Mr Barakat and Ms Mohammed were married in December, said Hicks complained about once a month that the two men were parking in a visitor's space as well as their assigned spot.
"He would come over to the door. Knock on the door and then have a gun on his hip, saying, 'You guys need to not park here'," said Mr Ahmad, a graduate student in chemistry at UNC-Chapel Hill.
"He did it again after they got married."
Both Hicks and his neighbours complained to the property managers, who apparently did not intervene. "They told us to call the police if the guy came and harassed us again," Mr Ahmad said.
A probable cause hearing is scheduled for March 4. Police said Hicks was co-operating.
Mr Abu-Salha said Yusor wore a Muslim headscarf and told her family a week ago that she had "a hateful neighbour".
"Honest to God, she said, 'He hates us for what we are and how we look'," he told The News & Observer of Raleigh.
Muslim Advocates, a civil rights organisation based in California, urged US attorney general Eric Holder to open a government inquiry into the killings.
"We cannot ignore the environment in which this incident took place," said Madihha Ahussain, of the group.