U-2506 + U-2518 + U-2519

Many of the U-Boats undergoing training, repairs or inspection in 
the Baltic Sea took part in the operations to evacuate to the west 
civilians and wounded soldiers from before the advancing Soviet 
armies. In the evening of 30.01., U-2518 (Kptlt. Friedrich Weidner), 
U-2519 (K.Kpt. Peter Erich Cremer) and U-2506 (Kptlt. Horst von 
Schroeter), sailing to Travemunde from the Gulf of Danzig, detected 
the Soviet submarine S 13, which sailed intermittently on the surface. 
Due to the ordered radio silence, they could not transmit that 
information. At 2116 hours, the Nibelung hydroacoustic equipment of 
U-2506 detected 3 explosions. They were the detonations of 3 torpedoes 
fired by S-13, which hit the German ship Wilhelm Gustloff. 
Approximately 4,500 people perished with the ship. 

Ref: Ubootwaffe, Marine - Kleinkampfverbande 1939-1945 by Waldemar Trojca