Witness Mohan Ibrahim said he was one of 200 people in the Masjid Al Noor mosque on Deans Avenue in Christchurch when he heard shots fired.
Ardern said one suspect was Australian-born and all had what she described as extremist views.
The government advised people not to go to mosques until further notice.
He visited Bulgaria from November 9-15 a year ago claiming he wanted "to visit historical sites and study the history of the Balkan country", Bulgaria's public prosecutor Sotir Tsatsarov said, adding that the inquiry would establish if this was "correct or if he had other objectives".
"Today we stand with the people of Christchurch, New Zealand".
It has been widely reported that the suspected gunman in the New Zealand massacre took to social media platforms to live stream his attacks.
In the video, the killer spends more than two minutes inside the mosque spraying terrified worshippers with gunfire.
Most guns do not require registration under New Zealand's Arms Act and police do not know "how many legally or illegally owned firearms there are in New Zealand", according to a police statement a year ago.
"Then people started running out". Children's screams can be heard in the distance as he returns to his auto to get another rifle.
After going back outside and shooting a woman there, he gets back in his vehicle, where the song "Fire" by the English rock band The Crazy World of Arthur Brown can be heard blasting.
"An expert testified before parliament in 2016 that it was only a matter of time before a mass shooting occurred in New Zealand", he told SBS News.
The footage showed the killer was carrying a shotgun and two fully automatic military assault rifles, with an extra magazine taped to one of the weapons so that he could reload quickly.
Separate "endorsements" are required to own semi-automatic weapons like those used in Friday's attack, as well as pistols and other restricted weapons. The gunman in question demonstrated ties to numerous extreme-right ideologies and individuals in his manifesto and livestream of the attack, during which he mentioned the YouTuber. He said his brother last visited Jordan two years ago.
New Zealand Cricket said they had chose to cancel the test, which was scheduled to start at Hagley Oval on Saturday, after discussions with the Bangladesh Cricket Board.
At least 49 people were killed and more were injured.
Before Friday's attack, New Zealand's deadliest shooting in modern history took place in 1990 in the small town of Aramoana, where a gunman killed 13 people following a dispute with a neighbour.
Christchurch was the home of these victims.