The man, identified as 21-year-old Philip Manshaus, appeared in the Oslo court with two black eyes and scrapes and bruises on his face, neck and hands, probably obtained when he was overpowered at the mosque.
Police say he has “extreme right views” and “xenophobic positions”. The Norway incident comes amid a rise in white supremacy attacks around the world.
Manshaus is formally suspected of murdering his 17-year-old stepsister, and of a terrorist act at the Al-Noor mosque on Saturday, allegations he has rejected.
In Norway, being formally named as a suspect is a step prior to indictment.
Manshaus entered the courtroom smiling to cameras, wearing dark clothes and his hair cut short.
He asked to be released, his lawyer Unni Fries said after the hearing.
“He rejects the allegations and exercises his right to not explain himself,” she said.
The court said there were “reasonable grounds” to suspect Manshaus had committed the criminal acts and remanded him in custody for four weeks, as police had requested.
During the first two, he will be kept in “complete isolation”.
'Pretty vague' past tip
Manshaus is accused of entering the mosque in the affluent Oslo suburb of Baerum armed with at least two weapons and opening fire before being overpowered by a 65-year-old man who suffered minor injuries. Just three worshippers were in the mosque at the time.
Hours after the attack, the body of a young woman was found in a home in Baerum and police on Sunday confirmed it was Manshaus' 17-year-old stepsister.
According to local media, she was of Chinese origin and had been adopted by the companion of Manshaus' father.
On Monday, Norway's domestic intelligence service PST said it had received a tip “about a year ago” about Manshaus, but that they chose not to act on it.
“The tip was pretty vague and was not indicative of any imminent terrorism plot,” PST chief Hans Sverre Sjovold told reporters.
PST, which said it receives a large number of similar tips, has not raised the threat alert level in Norway from the current low level.
'Race war' post
Shortly before the mosque attack, a person identifying himself as Philip Manshaus had posted a message on the EndChan forum calling for a “race war” to be taken from the internet into real life (“irl”).
The author said he was selected by “saint tarrant”, an apparent reference to the New Zealand mosque attack suspect Brenton Tarrant, accused of killing 51 people in attacks on two mosques in Christchurch in March.
EndChan confirmed on Twitter that a man “claiming to be the Oslo shooter” had posted material on the forum.
The post ended with the words “Valhalla awaits”, a reference to the afterlife for those who have died in battle in Norse mythology.
The shooting follows a rise in white supremacist attacks, including the recent El Paso massacre in the United States.
The Oslo attack took place on the eve of the Islamic celebration of Eid Al-Adha, marking the end of the Hajj pilgrimage, stoking fears among Norway's Muslims.
In Norwegian media, neighbours and acquaintances have meanwhile described the suspect as a previously happy and well-adjusted person whose behaviour had changed during the past year.
According to public broadcaster NRK, he reportedly became “very religious” and adopted increasingly extreme rightwing views.
Norway was the scene of one of the worst-ever attacks by a rightwing extremist in July 2011, when Anders Behring Breivik, who said he feared a “Muslim invasion”, killed 77 people in a shooting spree at a Labour Party youth camp on the island of Utoya and a truck bomb blast near the government offices in Oslo.