The U-boat commander, Kapitänleutnant Werner Hartenstein and his crew heard Italian voices coming from those struggling in the water and realized their error. The Germans immediately commenced rescue operations and were joined by the crews of other U-boats in the area.
At 10:22 p.m. the liner, sailing under the name Laconia, transmitted the following message on the 600-meter band
signifying "under attack by submarine".
As Laconia began to sink, Hartenstein surfaced. He hoped to capture the ship's senior officers. To his surprise, Hartenstein saw over two thousand people struggling in the water.
Survivor Jim McLoughlin states in One Common Enemy Hartenstein asked him if he was in the Royal Navy, which he was, and then asked why a passenger ship was armed, stating, "If it wasn't armed, I would not have attacked." McLoughlin believes this indicates Hartenstein had thought it was a troop transport rather than a passenger ship; in fact, by signalling to the Royal Navy, Laconia was acting as a de facto naval auxiliary. Moreover, merchantmen armed with guns (which most were) fell outside the protection from attack without warning and the requirement to place survivors "in a place of safety" (for which lifeboats did not qualify); therefore, it made no difference if she was a troop ship.
Head of submarine operations, Admiral Dönitz, immediately ordered two other U-boats to divert to the scene. Soon U-156 was crammed above and below decks with nearly two hundred survivors, including five women, and had another 200 in tow aboard four lifeboats. At 6am on September 13, Hartenstein broadcast a message on the 25-meter band in English (and plain language) to all shipping in the area giving his position, requesting assistance with the rescue effort and promising not to attack. It read:
U-156 remained on the surface at the scene for the next two and a half days. At 11:30am on September 15, she was joined by U-506 commanded by Kptlt. Erich Würdemann and a few hours later by both U-507 under Korvettenkapitän Harro Schacht and the Italian submarine Cappellini. The four boats, with lifeboats in tow and hundreds of survivors standing on their decks, headed for the African coastline and a rendezvous with Vichy French surface warships which had set out from Senegal and Dahomey.
Harden flew back to the scene of the rescue effort and at 12:32 p.m. attacked with bombs and depth charges. One landed among the lifeboats in tow behind U-156 while others straddled the submarine itself. Hartenstein cast adrift those lifeboats still afloat and ordered the survivors on his deck into the water. The submarines dived and escaped. Hundreds of Laconia survivors perished, but French vessels managed to re-rescue about a thousand later that day. In all, some 1,500 passengers survived.
Under the Hague Conventions, hospital ships are protected from attack, but their identity must be communicated to belligerents (III, 1-3), they must be painted white with a Red Cross emblem (III, 5), and must not be used for other purposes (III, 4). Since a submarine remained a military vessel even if hors de combat, the Red Cross emblem did not confer automatic protection, although in many cases it would have been allowed as a practical matter. The order given by Richardson has been called a possible war crime, but the use of a Red Cross flag by an armed military vessel would be a violation under the Geneva Convention of 1949 (II, 44). There is no provision in either convention for temporary designation of a hospital or rescue ship. Under the informal rules of war at sea, however, ships engaged in rescue operations are held immune from attack.
At the Nuremberg Trials held by the victorious Allies in 1946, Dönitz was indicted for war crimes, including the issuance of the "Laconia order":