Auckland (CNN) New Zealand's government has agreed to reform the country's gun laws in the wake of last Friday's massacre at two mosques, in which 50 people were killed, Prime Minister Jacinda Ardern has confirmed.

Ardern said that the "worst act of terrorism on our shores" had exposed a range of weaknesses in New Zealand's gun laws.

"Within 10 days of this horrific act of terrorism we will have announced reforms which will, I believe, make our community safer," she said.

While acknowledging that "for a short period" the planned amendments might create uncertainty for some gun owners, Ardern said: "I strongly believe that the vast majority of gun owners in New Zealand will agree with the sentiment that change needs to occur."

Earlier Monday, popular New Zealand e-commerce website TradeMe ended the sale of semi-automatic guns on its online marketplace.

"We have listened to public sentiment following Friday's terrorist attack in Christchurch and decided to remove all semi-automatic firearms sales and parts associated," TradeMe wrote in a statement.

New Zealand Prime Minister Jacinda Ardern confirms reforms to gun laws will be announced by March 25.

Inquiry to be launched into attack

The Prime Minister also announced that there would be an inquiry into the specific circumstances leading up to Friday's attack.

The inquiry will look into what agencies knew -- or should have known -- about the gunman's access to weapons or any impediments into the sharing of information, she said.

It will also look at the individual's travel movements, activities in New Zealand, use of social media and contact with others.

The key agencies the inquiry would look at include the New Zealand Security Intelligence Service, Government Communications Security Bureau, Police, Customs and Immigration, she said.

Ardern said that her government had also had preliminary discussions around ensuring that New Zealanders had an opportunity to commemorate the victims as one but that any national memorial service would not take place this week.

Ardern, accompanied by the governor-general, Patsy Reddy, and Parliament Speaker Trevor Mallard, on Monday opened a national condolence book for victims. People can go to the National Library in the capital of Wellington to sign the book.

"While it is a small action, the condolence book offers an opportunity for New Zealanders to unite and express our opposition to hate and state our commitment to the values of love and compassion," Ardern said.

First body of victim released

New Zealand Police earlier said that the first body belonging to a victim of Friday's massacre had been released to their family.

Authorities have been racing to identify the 50 people killed, with Islamic tradition calling for a person to be buried as soon as possible after death -- ideally within 24 hours.

A Syrian refugee, a Pakistani academic and their sons were among the 50 people killed, family members and non-profit organizations confirmed.

"We had our first release of a deceased person, official release last evening," Police Deputy Commissioner Wally Haumaha said during a news conference in Christchurch Monday afternoon local time.

However, Haumaha told reporters the family had declined to take possession of the body, choosing to wait instead for the release of a second family member who was also killed in the attack.

"The process has been very emotional and highly stressful for all and you can imagine the emotions are running high, as in accordance with the Islamic faith, the families have wanted their bodies to be returned as soon as possible. We are ensuring that we do that," he said.

Ardern said Sunday that authorities had started returning identified bodies to families, and all bodies would be returned by Wednesday. Six disaster victim identification experts have traveled from Australia to help hasten the process, she said.

Injured still critical

In addition to the people killed, 50 others were wounded in the attack, authorities said. Of the injured, 31 people remain in Christchurch Hospital, including nine in critical condition in intensive care, health officials said Monday.

A 4-year-old girl also remains in a critical condition at a hospital in Auckland after being transferred there on Saturday. Her father is also being treated in Auckland and officials said he was in serious but stable condition.

New Zealand police described efforts to identify the victims as "detailed and complex work" that must be completed thoroughly.

The victims' names were not made public but a preliminary list has been shared with families, police said Sunday.

New Zealand's largest criminal investigation

New Zealand Police Commissioner Mike Bush told reporters Monday that the investigation into the attacks was the largest criminal probe ever undertaken by the country's police.

He said 250 detectives and specialists were conducting the investigation, with jurisdictions all around the world assisting. The FBI, Australian Federal Police and the New Zealand Police Force's own partner agencies were also working on the ground in New Zealand, Bush said.

Three days after the shootings, Australian Brenton Harris Tarrant, 28, appears to be the only person in custody who has been linked to the attack.

"We believe that there is only one attacker responsible for this horrendous event," the commissioner said, adding that it was possible the suspect still could have received support from others.

Tarrant had been living in the southern city of Dunedin, around 225 miles from Christchurch and had traveled around the world, including Turkey and Pakistan.

Officials said he had no criminal history in New Zealand or Australia and had not drawn the attention of the intelligence community for extremist views.

Bush said the threat level in New Zealand remained high and there would be increased visibility from police and emergency service partners "for weeks to come."

Graphic video raises questions over offensive content

The attack was broadcast live on Facebook and the graphic video was copied and shared by users of the platform.

Facebook removed 1.5 million videos of the mosque attack in the first 24 hours, the social media company tweeted Sunday.

Of these, Facebook said more than 1.2 million were blocked at the point of upload.

Additionally, all edited versions of the video that don't show the graphic content were also removed "out of respect for the people affected by this tragedy and the concerns of local authorities," Mia Garlick of Facebook New Zealand, said on Twitter.

Friday's video has reignited questions about how social media platforms handle offensive content, with many questioning if companies are doing enough to try to catch this type of hate-filled content.

Tarrant also sent an 87-page manifesto to Ardern minutes before the attack.

The document, also posted on social media before the shooting, was filled with anti-immigrant, anti-Muslim screeds. Authorities have declined to discuss potential motives for the attack.

Tarrant, who is facing one murder charge, made a hand gesture associated with white supremacists when he appeared in court on Saturday.

He was remanded in custody and will reappear in court April 5. The duty lawyer temporarily assigned to the suspect told CNN Monday that Tarrant had made it clear that he did not want legal representation.

Richard Peters, assigned by Christchurch District Court to assist Tarrant in preliminary court appearances, said that the defendant hopes to represent himself.