Original Documents from the National Archives, Kew:

     The sinking of the LUSITANIA, 1915

     The British passenger ship LUSITANIA, 30,396 BRT, was sunk by the German submarine
     U-20 on 7th May 1915 south of Ireland - torpedoed without warning. 1,201 men, women
     and children were killed.

   ADM 137/4152 - U-boat history sheets comp. By Lt.Cdr. Tiarks :

     U-20 sunk the LUSITANIA on May 7, 1915. Sheet No. 4 shows the exact position
     of U-20 off the Irish South Coast, while the LUSITANIA is arriving from the West.
     Every 4 hours U-20 gave its position by W/T - deciphered immediately by the British Admiralty, Room 40.

    Operations Known to British Intelligence: U-20, 1914-1916, 17 pages

   ADM 137/4353 - Lusitania signals and copies :

     W/T messages, 6 pages  

   ADM 137/3923 - Photographic plates of log book of U-20 for May, including record of sinking of the LUSITANIA :

     War diary U-20, May 1915, 7 pages