"PST's conclusion is that Hassan Dhuhulow was one of the terrorists. The investigation has been concluded," Martin Bernsen, the head of information for Norway's Police Security Services (PST) told Norway's Aftenposten newspaper.
On 21 September 2013, a group of gunmen attacked the Westgate mall in an affluent area of Nairobi, indiscriminately shooting shoppers and staff.
The mall remained under siege for four days as security services attempted to hunt down the assailants, who remained inside and were finally killed, partly by police and partly by areas of the mall collapsing.
Somali terror group Al-Shabaab claimed responsibility for the attack.
Dhuhulow had been a suspect in the case since 2013, but his involvement had yet to be confirmed due to a lack of forensic evidence. He has now been identified by dental records.
"The reason is that several countries have been involved in the case. It's mainly the identification work that has taken a long time, and that has affected our progress," Bernsen said.
Prosecutor Jan Glent said that PST had been waiting for the United States Federal Bureau of Investigation (FBI) to complete it's own investigation.
"We have been waiting for a forensic investigation from the FBI. It has taken time, and that is the reason why this case has taken such a long time," he said.
Dhuhulow came to to Norway as a nine-year-old in 1999. He has been watched by PST ever since he joined a radical Islamic group as a teenager.