The suspect is accused of carrying out Saturday's shooting at the Al-Noor Islamic Centre mosque in Baerum, west of Oslo. Only three people were present at the house of worship when the suspect opened fire and one of the congregants was able to overpower him until police arrived.
The 17-year-old was adopted as a small child by a Norwegian woman, who is now the spouse of Philip Manshaus's father, the mother's lawyer said.
"We are looking at an attempted act of terror", acting chief of the police operation Rune Skjold told a press conference after Saturday's incident left one man injured.
According to Norwegian newspaper VG, police confirmed that she was the suspect's 17-year-old younger sister.
A man suspected to be the gunman at the Oslo mosque shooting has been arrested.
The attempted attack on al-Noor Islamic Center happened a day before Muslims around the world celebrate Eid al-Adha, one of the most important holidays on the Islamic calendar marking the end of the hajj pilgrimage. The shooter wore body armor and a helmet, police said.
Oslo police spokesman Roar Hanssen said in a statement: 'The Oslo police district tonight has found a dead person in a residence... in Baerum.
Shortly before the mosque attack, a person identifying himself as Philip Manshaus had posted a message on the EndChan forum calling on readers to move a "race war" off the web and into real life ("irl").
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"The investigation is still in an early phase and the suspect has not made any statements to the police", police said in a statement.
The suspect, whose exact age has not been released but has been listed as being 21 or 22-years-old, appeared in court with bruises across his face.
He told Reuters news agency: "I suddenly heard shouting from outside".
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 suspect's lawyer, Unni Fries, said in a brief phone interview on Monday that she could not comment on the case.
Norway's Prime Minister Erna Solberg called the shooting a "direct attack on Norwegian Muslims".
Head of Norway's security police (PST) Hans Sverre Sjovold speaks at a news conference in Oslo, Norway, on August 12, 2019.
In July 2011, Anders Behring Breivik, who followed right-wing ideology and said he feared a "Muslim invasion", killed 77 people in a truck bomb blast near government offices in Oslo and a shooting spree at a Labour Party youth camp on the island of Utoya.