Police in Norway said Sunday that an officer was supposed to be on the island where a mass shooting took place on Friday, but it is not clear where that officer actually was at the time of the attacks, Deutsche Welle reports.

At least 86 people were killed in the gun attack and authorities fear the death toll may rise as the search for further possible victims continues. It is feared some youths may have drowned as they attempted to escape the gunman by swimming from Utoya island.

The gunman apprehended at the scene by armed police was dressed as a police officer.

Acting Oslo police chief Sveinung Sponheim said in a press conference Sunday that it remains unclear where the officer who was due to be at the island was during the shooting.

Norwegian police have come under fire to explain why it took so long for the armed response teams to arrive on the island.

It may have taken up to 90 minutes for officers to arrive on the scene after the shooting started. The police response was hampered by the lack of access to a helicopter to reach that area and then a boat to access the island, the AP reports.

Sponheim also said Sunday that investigators are being assisted by a criminal technical expert from Britain, Reuters reports. The Metropolitan Police in the UK also confirmed that one of its officers was involved in the inquiry.

Norwegian authorities continue to question 32-year-old Anders Behring Breivik, who has been charged with terrorism offenses relating to both attacks. He claims to have acted alone and says his actions were "cruel" but "necessary".  

The overall death toll from the two attacks Friday rose to 93 Sunday after one of those injured in the shooting died from their injuries. Thousands of people attended memorial services Sunday alongside the Norwegian royal family and the prime minister to commemorate the victims of the twin attacks.

(Read more Global Post coverage on Norway: Norway massacre suspect admits responsibility)