Remaining ships retreat

By the time Blücher sank, the remaining naval force destined for Oslo had long since turned around and retreated back down the fjord. Seeing the geysers of water from underwater explosions on Blücher, and unaware of the torpedo battery, the commander of the heavy cruiser Lützow (the recently renamed pocket battleship Deutschland) assumed the flagship had hit mines and at 04:40 the decision was made for the flotilla to turn back and land the invasion forces out of range of the Oscarsborg batteries. The planned coup against Oslo to force the surrender of the Norwegian government was replaced by a land advance up the Oslofjord.

As the force made good its escape, the fortress managed to damage Lützow, the 15 cm guns of the Kopås battery scoring three hits and knocking out the ship\’s forward 28 cm turret (\»Anton\»). Kopås kept firing at the retreating ships until they disappeared in the mist at a range of around 3,000 m (3,300 yd). After pulling out of range of the fortress guns, Lützow employed her remaining turret \»Bruno\» to bombard the defenders from a range of 9–10 km (4.9–5.4 nmi; 5.6–6.2 mi) down the fjord.

During the battle, another burning ship was spotted in the distance from Oscarsborg, leading the Norwegian defenders to believe they had sunk another German warship in addition to Blücher. For some time after the battle, the belief was that Oscarsborg had sunk the artillery training ship Brummer. The reality, however, was that the burning ship was the 107 long tons (109 t) Norwegian cargo cutter Sørland that had stumbled into the battle while on her way from Moss to Oslo with a cargo of paper. As the small ship had mistaken the events as a military exercise, she kept going until fired upon and set ablaze by the German minesweepers R-18 and R-19. Sørland sank with two of her six-man crew near the village of Skiphelle in Drøbak, as the first civilian Norwegian ship lost during the invasion. Brummer was indeed lost in connection with the invasion, but only when she was on her way back to Germany on 14 April, when she was torpedoed by the Royal Navy submarine HMS Sterlet and sank the next day.