SOFIA, Bulgaria (AP) — Bulgarian military prosecutors on Thursday indicted a group of serving and retired military officials on charges of spying for Russia.

The indictment filed in a military court accuses six people of collecting and passing classified information to a foreign country, the prosecutors said. They include a former senior official in Bulgaria's Military Intelligence Service, senior Defense Ministry officials and a former military attaché who oversaw classified information at Bulgaria’s parliament.

The six were arrested in March; five of them remain in custody while one was released on bail. A preliminary hearing has not been scheduled yet. The six could face sentences from 10 years to life in prison, if convicted, prosecutors said.

Prosecutors charged the former Military Intelligence Service official as the ringleader of the plot. He graduated from the intelligence school in Moscow run by Russia’s GRU foreign military intelligence, authorities said.

The prosecutors allege that upon his return to Bulgaria, he was tasked with recruiting a network of agents with access to classified documents linked to NATO and the European Union. His wife, who holds dual Bulgarian-Russian citizenship, handed over the documents at the Russian Embassy in Bulgaria.

Last year, Bulgaria expelled five Russian diplomats accused of spying who could not be charged because of their diplomatic immunity. Among them was Russia’s military attaché, whom prosecutors allege coordinated the spying network in Bulgaria.