At a news conference on Monday, police shed no light on what may have prompted the attack -- or whether a second shooter was still at large.
Canadian Prime Minister Justin Trudeau has condemned as a "terrorist attack" Sunday night's assault on the Islamic Cultural Center in a busy district of Quebec City, which sent terrified worshippers fleeing barefoot in the snow.
Police said one suspect -- reportedly a Canadian student -- was in custody after surrendering to the authorities, while a second person had been questioned as a witness.
But at a news conference on Monday, police shed no light on what may have prompted the attack -- or whether a second shooter was still at large.
Both police and witnesses had initially described two masked men open fire inside the mosque, where about 50 worshippers were gathered for evening prayer. Of the eight people wounded, five remained in critical condition in hospital.
According to eyewitnesses who spoke to Radio-Canada, one had a "strong Quebecois accent."
Some 50 people were in the mosque at the time. The dead are all dual nationals.
Police responding to reports of shots being fired arrested a suspect at the scene at 7:55 pm (0055 GMT Monday).
Another man -- described as 25 to 30 years old -- surrendered 20 kilometers (12 miles) outside the city after calling an emergency services hotline an hour after the attack to reveal his location.
According to local media, the man who turned himself in is a Canadian political sciences student at Laval University named Alexandre Bissonnette.
The second person detained, a Canadian student of Moroccan origin named Mohamed Khadri, was questioned as a witness and has since been released, reports said.
Royal Canadian Mounted Police Inspector Martin Plante told an earlier news conference that neither detainee had any apparent foreign links.
"It is a domestic investigation at this time," Plante said.
The Quebec mosque had already been the target of hate: a pig's head was left on the doorstep last June during the Muslim holy month of Ramadan.
Other mosques in Canada have been targeted with anti-Muslim graffiti in recent months.
Police stationed near the mosque told AFP that they had feared this type of attack "because it's happening all over the world."
The shooting came as Canada vowed to open its arms to Muslims and refugees after US President Donald Trump's controversial immigration ban prompted travel chaos and outrage around the world.
Trudeau, who was to travel to Quebec City in the afternoon, issued a statement saying "we condemn this terrorist attack on Muslims in a center of worship and refuge."
"Muslim Canadians are an important part of our national fabric, and these senseless acts have no place in our communities, city and country," he said.
"It is heart-wrenching to see such senseless violence," he added. "Diversity is our strength, and religious tolerance is a value that we, as Canadians, hold dear."
Trump telephoned Trudeau to offer his condolences following the attack, which was condemned around the world including by leading Sunni Muslim body Al-Azhar, as well as Egypt and Jordan.
Mohamed Ali Saidane lost a friend in the shooting: a father of three who he said "stood up and tried to stop the first shooter."
Saidane joined elected officials at city hall Monday to denounce escalating hate.
"The Arab-Muslim community has become the victim of populist politics," he said. A Tunisian university professor was also among the dead.
"It's terrible for the community, it's terrible for Quebec, it's terrible for us to live together," he told AFP.
People fleeing the mosque -- where custom requires worshippers to remove their shoes during prayer -- did not have time to collect their winter boots.
"They arrived in a panic," said Louis-Gabriel Cloutier, the manager of a cafe across the street who saw them fleeing the rampage.
Some took refuge at the cafe.
"I never thought that such a thing could happen," said a man who frequents another of the city's 10 mosques. He had ventured to the scene because "I know people who were inside."
Hamid Nadji learned of the shooting from a friend and rushed to the mosque area.
"For us Muslims, Quebec and Canada had been a safe zone," he said.