"We have this video. It gives the police important evidence", Oslo district police attorney Paal-Fredrik Hjort Kraby said at a news conference on Monday, the NRK broadcaster reported.
At least one person was injured in the attack, while police affirmed that the suspect used a GoPro camera.
A man, later identified as Philip Manshaus, opened fire on the Al-Noor Islamic Centre in the municipality of Baerum near Oslo on Saturday evening.
The incident happened as Muslims across the world are preparing to celebrate Eid al-Adha, one of the most important holidays in Islam.
Oslo police specified that the man was acting alone. He was overpowered by mosque clergy who then called police.
Police later found a dead woman in his house. Local media reported that the victim was his stepsister.
Norwegian intelligence received tips as far back as last year about the shooter, according to Norwegian Police Security Service (PST) Chief Hans Sverre Sjovold. Sjovold said, however, that tips received by the intelligence services were unclear and unrelated to the planning of the attack.