Manshaus, whose home is near the mosque just outside the Norwegian capital, had expressed far-right, anti-immigrant views before the attack, police said.
The man suspected of shooting at people inside a Norwegian mosque Saturday, and of killing his stepsister, appeared in court Monday with black eyes and wounds on his face and neck.
A judge gave police permission to hold 21-year old Philip Manshaus in custody for an initial four weeks while he is investigated on suspicion of murder and breach of anti-terrorism law, the court's ruling later showed.
Manshaus does not admit to any crime, his lawyer said earlier.
Eyewitnesses said Manshaus entered the al-Noor Islamic Centre with several guns, but was overpowered by a 65-year-old member of the mosque, who managed to wrestle away his weapons in the fight that followed.
A few hours after the mosque attack, police discovered the body of a young woman at what they said was the suspect's address. Police later said she was his stepsister.
Manshaus did not speak while reporters were present, and has so far declined to talk to the police.
"He is exercising his right not to be interrogated," his defense attorney, Unni Fries, told Reuters. "He is not admitting any guilt."
Manshaus, whose home is near the mosque just outside the Norwegian capital, had expressed far-right, anti-immigrant views before the attack, police said earlier.
Online postings under Manshaus' name, made shortly before the attack, expressed admiration for the massacre at two New Zealand mosques in March by a suspected right-wing extremist, in which more than 50 people were killed. Norwegian media also reported that he had also inspired by the New Zealand attacks as well as the recent El Paso shooting in the United States which took place on Aug. 2. Manshaus' lawyer declined to comment on such reports.
Police sought to hold him on suspicion of murder, as well as of breaching anti-terrorism law by spreading severe fear among the population.
"The investigation is still in an early phase and the suspect has not made any statements to the police," prosecutors said in a statement.
The Norwegian police security service, PST, which monitors and investigates extremist threats, said on Monday it had received a tip-off regarding Manshaus last year but had not launched an investigation at the time.
"There was nothing in that tip-off that suggested there was danger of an act of terrorism or that planning of an attack was underway," PST head Hans Sverre Sjoevold told a news conference.
PST will now explore whether Manshaus had links to any domestic or foreign extremist networks, although investigators have said they believed he acted alone in the attack, Sjoevold told Reuters.
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.
Norwegian Prime Minister Erna Solberg on Sunday said while her government tries to combat hate speech, more must be done. "We are trying to combat this, but it's a challenge. I think it's a word-wide challenge in a sense," Solberg said.
Any formal charges in the case, and a trial to decide whether Manshaus is guilty or not, are likely to still be months away.
A guilty verdict on charges of breaching anti-terrorism laws can carry a sentence of up to 21 years in prison, as can the killing of the suspect's 17-year-old stepsister, according to Norwegian sentencing guidelines.