The messages from a private WhatsApp group, which included retired high-ranking members of the air force, were published by the Infolibre news website earlier this month, sparking outrage.

The chat participants regularly railed against Socialist Prime Minister Pedro Sanchez's minority coalition government with far-left party Podemos, which has governed since January.

It is Spain's first coalition government since the country returned to democracy following the death of longtime dictator General Fransisco Franco in 1975.

"I don't want these scoundrels to lose the elections. No. I want them and all of their offspring to die," one chat participant wrote.

"For them to die, they must be shot and 26 million bullets are needed," another chat participant wrote, referring to the number of Spaniards who voted for the parties making up the government or supporting it.

The messages frequently praised Franco and one wrote that "someone's going to have to start doing something (legal or illegal) against these sons of bitches."

In a statement, the Madrid public prosecutor's office said the WhatsApp group "contains statements totally contrary to the constitutional order, with veiled references to a military coup."

According to Infolibre, some of the WhatsApp group members were among the over 70 former military officers who wrote to King Felipe VI last month to criticise Sanchez's "social-communist" government.

The retired officers also accused the government of provoking the "breakdown of national unity" with its deals with Catalan and Basque separatist parties in parliament.

This particular complaint echoes those of Spain's conservative and far-right parties.

Defence Minister Margarita Robles had called for an investigation into the leaked chats, saying they "did not represent" Spain's armed forces.