Judge Jose de la Mata of Spain's National Court, which handles terrorism cases, said he sent a rogatory letter in May 2015 to judicial authorities in the United States to request that Microsoft and Facebook save the e-mails and social media profiles of the six men.
"For months, conversations and meetings with various representatives of US authorities were held, additional information was provided to them, and the urgency and relevancy of the request was repeatedly reiterated," he wrote in a summary of his investigation.
"Despite all this, it was impossible to get the required international cooperation from the United States. All the requests carried out have been unsuccessful, and there is no reasonable expectation that the petition for international judical cooperation will be met in the future."
Police detained the six men, including two pairs of brothers, in the tiny Spanish territory of Ceuta, which is located across the straight of Gibraltar from mainland Spain and shares a land border with Morocco.
At the time, Interior Minister Jorge Fernandez Diaz compared the suspects to the two brothers who had killed 12 people in an attack on the Paris offices of the French satirical magazine Charlie Hebdo earlier that month.
The suspects detained in Ceuta also had a "firm determination... to commit an attack and if necessary sacrifice themselves and die in the attempt," he said.
Police seized a nine-millimetre automatic pistol and machetes among other items during dawn raids on two properties in Ceuta as part of the operation.
Spain's interior ministry said the suspects "formed part of a jihadist cell" that was inspired by online messages from the Islamic State (IS) group, which controls parts of Iraq and Syria.
The six suspects are under official investigation for cooperation with a terrorist group, praising terrorism and of attempting to recruit jihadists.