12 September 1948,
The attack on Franco went wrong on the Concha races. Several members of the CNT, with an aircraft departing from Dax, were about to release 120 kilos of explosives to the head of the dictator who was watching the sport test on his boat. For a few minutes, the plane traveled over Concha Bay, but the military forced him to divert. The aggressors wanted to keep the material and bomb Aiete's palace the same day, but they couldn't do a storm.
Source:
Angel of the Blonde, How to kill a dictator. (Ed. Melusina)