Cesar Montana Lehman, leader of the Madrid-based band Def Con Dos who goes by the stage name Cesar Strawberry, was arrested in May 2015 along with 18 other people for allegedly using social media to praise past attacks carried out by the groups and calling for fresh attacks against officials.
In a tweet dated December 20th, 2013 Lehman asked "how many more should follow the flight of Carrero Blanco?", in a reference to Admiral Luis Carrero Blanco, a Franco-era prime minister who was killed in 1973 in an Eta car bomb attack in Madrid.
The massive explosion sent Blanco's car hurtling into the air and over the roof of a church where he had just been attending mass.
In another tweet dated January 5th, 2014 Lehman joked about giving Spain's former King Juan Carlos a bomb for his birthday.
Lehman, 52, was charged with glorifying terrorism and humiliating victims of terrorism with his messages and prosecutors had asked that he be sentenced to 20 months in jail.
But Madrid's High Court ruled that it had not been proven that Lehman had sought "with these messages to defend the tenets of a terrorist organisation, nor humiliate its victims, in a ruling dated July 18th which was published on Tuesday.
During his trial Lehman acknowledged writing the Twitter messages but denied his intention was to support any form of violence or to humiliate victims of extremists attacks.
None of the groups cited in his Twitter messages has carried out attacks in recent years.
The largest of them, Eta, is blamed for more than 800 deaths in a four-decade campaign of bombings and shootings for an independent homeland in northern Spain and south-western France.
Eta declared a permanent ceasefire in 2011 but refuses to dissolve and turn over its arms as demanded by the governments of France and Spain.
Def Con Dos makes many references to pop culture, news and cinema in its lyrics. Spanish director Alex de la Iglesia used music by the band in his 1993 black comedy "Acción Mutante", or "Mutant Action", about a terrorist group made up of disabled people.