(Xinhua) -- Norway has raised strong complaints against the decision by Afghan authorities to release the five Afghan bombers who attacked the NATO-led International Security Assistance Force (ISAF) in Northern Afghanistan in May this year. "It just gives a wrong signal that people who have been sentenced for such acts are allowed to walk free," Norwegian Foreign Minister Jonas Gahr Stoere said in a statement. Stoere added that an explanation is needed of how it is possible for someone who was first sentenced to death to have the sentence reversed to 20 years in jail, only to be released because of a religious festival. The five Afghan bombers were sentenced to 20 years in jail, but Afghan President Hamid Karzai signed a decree to release them after they spent just three months in prison, Norwegian broadcaster NRK reported. The bomb attack killed one Finnish and one Afghan soldier, and injured three Norwegian soldiers.