Mr Manshaus is accused of opening fire at the Al-Noor Islamic Centre in Baerum, west of the capital Oslo, on Saturday.
They said the alleged perpetrator is also a murder suspect in a separate case.
Oslo's acting chief of the police operation Rune Skjold said the investigation into the shooting showed that the suspect, who has not been named, appeared to hold "far-right" and "anti-immigrant" views.
Hours after the attack, the body of a young woman was found in a home in Baerum and police on Sunday confirmed it was the suspect's 17-year-old stepsister.
Following Saturday's attack the police have increased security around the celebration of the Muslim holiday of Eid.More news: Friends Is Heading to the Big Screen for 25th Anniversary Celebration
Police sought to hold him on suspicion of murder, as well as of breaching anti-terrorism law by spreading severe fear among the population when firing several guns at the mosque.
Retired Pakistani Air Force officer Mohammad Rafiq, 65, approached the attacker first, the mosque said.
Skjold said the alleged gunman was prepared to cause deaths and more injuries at the Al-Noor Islamic Center and didn't succeed because of the "great courage" of people inside the mosque.
The Norwegian police security service, PST, said yesterday that it had received a tipoff about Manshaus a year ago, but did not investigate further.
His lawyer Unni Fries this morning declined to comment on Norwegian media reports that the suspect was inspired by shootings in New Zealand, where a gunman killed 51 people in March, and on August 3 in El Paso, Texas, which left at least 22 dead.More news: Sajid Javid maintains economy 'strong' despite seven year slump
No one was killed and one person was injured in the attack.
Conservative Prime Minister Erna Solberg on Sunday assured Norway's Muslim community of government support.
"For so many years, the secret police says the Muslims are the biggest risk for this country, but if you look at those last two major incidents of terrorist activities, it's not Muslims who have done this".
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").
Oslo police wrote on Twitter: 'There has been a shooting episode inside the mosque'. "We are trying to combat this, but it's a challenge", she said.More news: Perisic poised to join Bayern, reveals Kovac
In 2011, white supremacist Anders Behring Breivik massacred 77 people - the majority of whom were teenagers - in Norway's worst ever peacetime atrocity.