The shooter, described as a "young white man," wore body armour and a helmet, witnesses said.
The man suspected of shooting at people in a Norwegian mosque on Saturday and of killing his stepsister prior to the attack has denied carrying out the crimes and is not speaking with investigators, his lawyer said on Monday.
"He, for the moment being, is using his right not to give a statement to the police or to the court," his defence attorney, Unni Fries, told Euronews. She also confirmed that he denies guilt for the crime.
Philip Manshaus, 21, is also suspected of killing his stepsister, 17. The Oslo District Court ruled on Monday that he is to remain in detention for the next four weeks while the cases are being probed.
Norwegian police said on Sunday that it was investigating the previous day's shooting at the al-Noor Islamic Centre near the country's capital as "a possible act of terrorism." One person was injured during the attack.
A young woman was found dead at the suspect's address," assistant chief of police Rune Skjold also told a news conference, adding that the man was suspected of murder.
Earlier police had said that a "young white man" had been apprehended, adding that members of the congregation overpowered the gunman and stopped the shooting.
Police believe the man had acted alone.
"He is around 20 years old, a Norwegian citizen from the area," Skjold told Reuters.
A 65-year-old member of the congregation sustained light injuries in the attack but it was too early to tell if they were caused by the gunshots or sustained during the attempt to restrain the gunman, police said.
"The man carried two shotgun-like weapons and a pistol. He broke through a glass door and fired shots," mosque director Irfan Mushtaq told TV2.
The shooter, who wore body armour and a helmet, was overpowered by members of the mosque before police arrived, Mushtaq added.
There were only three people present in the mosque at the time of the attack, who were preparing for Sunday's Eid celebrations, which up to 1,000 people were expected to attend, mosque spokesman Waheed Ahmed told Reuters.
The mosque earlier this year implemented extra security measures following the massacre of more than 50 people at two New Zealand mosques by a suspected right-wing extremist.
In 2011, anti-Muslim neo-Nazi Anders Behring Breivik massacred 77 people in Norway's worst peacetime atrocity, the majority of them teenagers at a youth camp.