It's strongly suggested by some evidence that the CIA helped on this, they supplied the explosive, I recall that ETA never used this kind of plastic explosive but only here, it was the type os explosive that the CIA use at the time, and the tunnel they had to dig was too close to the USA Embassy, they had to know someone was digging a tunnel that close.
Also as Spain has 1/4 of the population that are fascists, you can't do jokes about this: Basque Space Launch, or the 1st spanish astronaut", because the Audiencia Nacional will sue you, the fascists rates in the judges are way higher than the 1/4 average.
Yes, evetually you will not be indicted, but they will sue you until you bleed
https://www.eldiario.es/politica/carrero-blanco-condenado-carcel-cassandra_1_2248138.html
Hence my use of "suggested", unless someone in the future declassify some documents and there is proof it's all speculations, USA didn't want more fascists in Europe and Carrero Blanco was the number one successor of Franco, he also was not very happy with the US military bases in Spain, and he want to re-negotiate it.
It's being said that the explosive is C4, military grade, that's why the car blew all the way up and pass the building, but at this moment there is no proof either way.
The fascist party (VOX) got almost 10% of votes last elections. The right-wing party (PP) got almost 40%.
Not all the fascists vote VOX though, many vote PP. So the amount of fascists is somewhere between 10% and almost 50%. Depending on how fascist you deem PP. 25% is not too far fetched.