On 11 March Gripenberg suggested truce of six weeks which the Russians Major General Muchanoff promptly rejected but agreed for a truce of three days after which hostilities would continue anew. Gripenberg agreed to a surrender on 14 March before the truce had run out. On 17 March Russian General commanding the armies in Finland, Friedrich Wilhelm von Buxhoeveden, accepted the surrender. Swedes surrendered the fortress with all its weapons intact to the Russians who in turn agreed just to dismiss the mostly Finnish garrison of Svartholm. The reasons for the capitulation are somewhat unclear but it seems like the Swedish officers didn't believe in the Swedish capability, nor the fortress' capability to withstand the Russians in this war. As many of the other Swedish officers, Gripenberg entered Russian service after the capitulation. He was labeled a traitor in Sweden and was sentenced to death, along with other officers, for the loss of Finland. Due to a general amnesty, the death penalty process was interrupted, also for him.