Quebec City, Canada - Canadian police have said only one of two men initially held in connection with a Quebec City mosque shooting - which killed six people - is now considered a suspect.

The second man, who was initially reported as a second suspect, was now considered a witness, provincial police said on Monday.

Authorities did not provide additional information about the identity of the suspect, or about the possible motive behind the shooting that left six worshippers dead and at least eight wounded at the Islamic Cultural Centre.

Another 12 people attending evening prayers sustained minor wounds, according to a local hospital spokesperson.

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The attack shocked people across Canada and sent ripples through the country's Muslim community.

Justin Trudeau, Canada's prime minister, condemned the shooting as a "terrorist attack on Muslims in a centre of worship and refuge".

"To the more than one million Canadians who profess the Muslim faith, I want to say directly: We are with you. Thirty-six million hearts are breaking with yours," Trudeau, who was expected to head to Quebec City later on Monday, said in the House of Commons in Ottawa.

In Quebec City, Muslim leaders called for unity and greater security for their community in the aftermath of the shooting.

"I can't express the great pain that touched our community with this tragedy that took place in a place of worship, against people who were praying," Mohamed Labidi, president of the Islamic Cultural Centre, said during a press conference on Monday morning.

"We are touched by this solidarity … and it diminishes our pain," Labidi said, breaking down in tears at the podium, where he was flanked by Quebec City Mayor Regis Labeaume and Quebec Premier Philippe Couillard.

"It hit us like a freight train. How could this happen in Quebec?" Labeaume said.

Couillard said the attack was a "terrorist act" that specifically targeted the Muslim community.

"We are all Quebeckers. All of us, a great people and a great nation, are united today," he said.

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Hate crimes targeting Muslim-Canadians more than doubled between 2012 and 2014 - a period in which such offences as a whole fell across the country.

Last year, the Islamic Cultural Centre in Quebec City was the target of an Islamophobic attack. A pig's head was left on the centre's doorstep, alongside a note that read, "bon appetit". Pork is forbidden in Islam.

The National Council of Canadian Muslims expressed outrage at Sunday's attack and called for the country to come together in the face of senseless violence and hatred.

"We are heartened by the overwhelming support from fellow Canadians in this time of deep crisis. We must unite together against divisive forces that seek to harm our communities," said Ihsaan Gardee, the national council's executive director in a statement.

The NCCM, a national lobby group that tracks anti-Muslim hate crimes, called on police to increase security around mosques and Islamic centres across Canada following the attack.

"Quebec Muslims are frightened right now," said Haroun Bouazzi, president of Montreal-based human rights group AMAL-Quebec. "We are urgently waiting for answers as to how and why such a tragedy could occur."

Labeaume said a vigil would take place in Quebec City on Monday evening, and another meeting is set for Tuesday to discuss steps moving forward, including additional safety measures for the community.

The Quebec flag was flying at half-mast in front of the provincial legislature, and the government has also set up an online form where the public can send their condolences to the victims of the attack.