At 07.17 hours on 14 May 1942 the unescorted and neutral Potrero del Llano (Master Gabriel Cruz Díaz) was torpedoed and sunk by U-564 east of Cape Florida. The survivors were picked up by the American patrol vessel USS PC-536 and taken to Miami.
Prior to the attack, Suhren had noticed an illuminated flag painted on the side of the ship and the identification books indicated that the ship had to be Italian, but this was not possible due to the position and route of the vessel so he decided to sink the tanker. The Mexican flag has the same colors like the Italian but has an eagle in the central part. The problem was that only ships of the Mexican Navy were permitted to show the eagle on the flag, the Mexican merchants were only allowed to paint the flag without eagle on the side of their ships, this resulted in the misidentification of their nationality.

On 1 June 1942 Mexico declared war on Germany after two Mexican tankers had been sunk by U-boats: Potrero del Llano by U-564 on 14 May and Faja de Oro by U-106 (Rasch) on 21 May.