Nearly 50 worshippers at two mosques in New Zealand were gunned down during Friday prayers in what the country's prime minister called "an extraordinary and unprecedented act of violence."
"This is one of New Zealand's darkest days," said Prime Minister Jacinda Ardern, who described the shootings as a terrorist attack that appeared to have been well-planned.
One man was in custody, charged with murder, and he appeared to have livestreamed much of the attacks on Facebook.
Police Commissioner Mike Bush said 49 people were killed with many more wounded. Scores of family members were awaiting news about missing loved ones, including children, at a local hospital.
New Zealand mosque shooting: What we know about shooterMarch 15, 201902:39
Officers responded to reports of shots fired around 1:40 p.m. Friday (8:40 p.m. ET Thursday) in Christchurch, a city of around 375,000 people. The two mosques are about three miles apart, and the second mosque was attacked about 45 minutes after the first.
Bush said that four people — three men and a woman — had initially been detained, but it remained unclear whether all of them were involved.
Police sources told NBC News' Australian partner Channel 7 that the main suspect's name is Brenton Tarrant, 28.
According to Bush, "a number of IEDs attached to vehicles" were also found. He said firearms had been recovered from both scenes.
"I cannot forget the sound of shooting."
The man who was charged didn't appear on any terrorism watchlists, authorities said. Bush said that "no agency had any information" about the suspects before the incident.
Australian Prime Minister Scott Morrison said that he was advised that the main suspect is an Australian national, and he condemned the attack "by an extremist right-wing violent terrorist."
Various social media posts on Twitter and the fringe message board 8chan were published online by someone who appeared to have had prior knowledge of the shootings, including pictures of weapons and ammunition.
Authorities have not confirmed that the postings are connected to the shootings, but pictures and videos seen by NBC News as well as a manifesto posted shortly before the attack match known details of the shootings.
The manifesto includes anti-Muslim rhetoric, far-right and far-left political sentiment and references to radicalization via the internet. It also addressed President Donald Trump.
"Were/are you a supporter of Donald Trump?" the author of the manifesto wrote. "As a symbol of renewed white identity and common purpose? Sure. As a policy maker and leader? Dear god no."
It also made reference to the struggle over gun control in the U.S.
"This conflict over the 2nd amendment and the attempted removal of firearms rights will ultimately result in a civil war that will eventually balkanize the US along political, cultural and, most importantly, racial lines," the manifesto said.
Users on one of 8chan’s message boards, where the manifesto and terror threat were posted before the shooting, frequently share racist, homophobic and misogynistic memes and diatribes.
Len Peneha who lives near one of the scenes said he saw a man dressed in black enter the Masjid Al Noor mosque and then heard dozens of shots, followed by people running away, The Associated Press reported.
Peneha told the AP that he went inside the building to help and "saw dead people everywhere.” He said the gunman ran out of the mosque, dropped what appeared to be a semi-automatic weapon and fled.
Mohan Ibrahim said he was among around 200 people inside that mosque.
New Zealand mosque shooting: Eyewitness describes horrorMarch 15, 201902:17
"When I heard the first sound I thought maybe there's an electric short circuit, but later on it was continuously a big sound, a loud sound was happening," he told NBC's "Today" show. "The windows and glass were dropping and breaking. At that time lots of people we were inside."
Ibrahim said he managed to escape through a door normally used by female worshippers at the back of the building.
"I had to run out of the area and I had to jump a wall," he said.
"I cannot forget the sound of shooting and the scenario that I have seen with my eyes," Ibrahim told Britain's Sky News. "I'm just shaking."
Ardern highlighted that the attack appeared to be targeting religious freedom, adding that the Muslim victims "should have been in a safe environment."
She also acknowledged that some of the victims "may even be refugees."
Ardern added: "Christchurch was the home of these victims. For many this may not have been the place they were born. In fact for many, New Zealand was their choice."
Of the potential suspects, she said, "These are people who I would describe as having extremist views, that have absolutely no place in New Zealand and in fact have no place in the world."
From 2014 through January, a total of 4,333 refugees arrived in New Zealand, according to government statistics. They mainly came from Syria, Myanmar and Afghanistan. In addition, 1,836 people claimed asylum in the country over the same period. New Zealand has a population of around 4.9 million.
Ardern also had a message for those who perpetrated the attack: "You may have chosen us but we utterly reject and condemn you."
New Zealand requires gun owners to have a firearms license, which can be applied for by people as young as 16 years of age through their local police station.
Applicants must take a firearms safety test and are required to follow regulations on safe storage for any firearms they own.
Around 225,000 people have such licenses and own an estimated 700,000 to 1 million firearms, according to the New Zealand Council of Licensed Firearms Owners.
New Zealand is not known for gun violence and there hasn’t been a terrorist incident there for decades, according to Mark Briskey, a senior lecturer in criminology at Murdoch University in Perth, Australia.
"Weapons have been associated, in the New Zealand context, with hunting," he said.
London Mayor Sadiq Khan responded to the shooting by expressing his condolences and trying to reassure members of his city’s large Muslim community.
In the U.S., police departments including in San Francisco, Los Angeles and New York City, said they would be making extra passing calls to mosques or assessing security in those cities as a precaution.