CHAPEL HILL: Police on Thursday were trying to determine whether hate played any role in the killing of three Muslims a day earlier, a crime they said was sparked by a neighbour's long-simmering anger over parking and noise inside their condominium complex.

Craig Stephen Hicks, 46, describes himself as a “gun toting” atheist. Neighbors say he 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.

The killings are fueling outrage among people who blame anti-Muslim rhetoric for hate crimes. A Muslim advocacy organisation pressed authorities to investigate possible religious bias. Many posted social media updates with the hashtags (hash)MuslimLivesMatter and (hash)CallItTerrorism.

About 2,000 people attended a candlelight vigil for the victims in the heart of UNC's campus Wednesday evening.

“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 in an email. His current wife, Karen Hicks, said he “champions the rights of others” and said the killings “had nothing do with religion or the victims' faith.”

Later Wednesday, she issued another statement, saying she's divorcing him.

Hicks appeared in court on Wednesday on charges of first-degree murder in the deaths of Deah Shaddy Barakat, 23, his wife Yusor Mohammad, 21, and her sister Razan Mohammad Abu-Salha, 19. He pleaded indigence and was appointed a public defender.

Officers were summoned by a neighbour who called police reporting five to 10 shots and the sound of people screaming. The women's father, Mohammad Abu-Salha, said police told him each was shot in the head inside the couple's apartment, and that he, for one, is 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 Abu-Salha, who is a psychiatrist.

Chapel Hill Police asked the FBI for help in their probe, and Ripley Rand, the US Attorney for the Middle District of North Carolina, said his office was monitoring the investigation. But Rand said the crime "appears at this point to have been an isolated incident".

Barakat and Mohammad were newlyweds who helped the homeless and raised funds to help Syrian refugees in Turkey this summer. 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. Mohammad planned to join her husband in dentistry school in the fall. Abu-Salha was visiting them Tuesday from Raleigh, where she was majoring in design at North Carolina State.

Imad Ahmad, who lived in the condo where his friends were killed until Barakat and 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 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 didn't intervene. "They told us to call the police if the guy came and harassed us again," Ahmad said. The killings were "related to long-standing parking disputes my husband had with various neighbours regardless of their race, religion or creed," Karen Hicks said.

A probable cause hearing is scheduled for March 4 and police said Hicks was cooperating.