"A young woman was found dead at the suspect's address," assistant chief of police Rune Skjold told a news conference late on Saturday, adding that the man was suspected of murder.
Police earlier said a "young white man" had been apprehended following an armed attack at the al-Noor Islamic Centre near the country's capital, Oslo, and members of the congregation managed to overpower the gunman and stop the shooting.
An older man sustained light injuries in the attack but it was too early to say if the wounds were caused by the gunshots or sustained during the attempt to restrain the gunman, according to police
An initial statement had said one person was shot in the attack.
The suspected attacker appeared to have acted alone, police said.
"He is around 20 years old, a Norwegian citizen from the area," Skjold told Reuters.
"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.
Only three people were present in the mosque at the time of the attack, preparing for Sunday's celebration of the Eid-al-Adha festival, which up to 1000 people had been expected to attend, mosque spokesman Waheed Ahmed told Reuters.
The mosque earlier this year implemented extra security measures following an attack on two mosques in Christchurch, New Zealand, in which 51 people died. Am Australian suspected right-wing extremist is facing multiple charges.
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.
