Who is Fethulah Gulen and why does Turkey's president want him in prison?

Muslim cleric Fethullah Gulen has been described as Turkey's second most powerful man.

The ageing cleric has a large following in Turkey, possibly stretching into the millions, who he reaches from his Pennsylvania base.

Gulen exiled himself in the United States in the late 1990s.

He is a former ally of Turkey's President Recep Tayyip Erdogan, who accused Gulen of trying to overthrow him last year.

Erdogan was prime minister at the time when a series of leaks surfaced in the Turkish media relating to an investigation into corruption within Erdogan's inner circle of political allies. The revelations led to the resignation of three government ministers.

For his part, Erdogan launched a massive purge of the police and judiciary, paranoid that Gulen's people had infiltrated vast parts of the state.

Prosecutors subsequently dropped the corruption investigations but Erdogan then stepped up a campaign against his long-time political rival Gulen. The then-prime minister said Gulen ordered his followers in Turkey to go after Erdogan and his allies.

Gulen was painted as the head of a rival state structure operating within Turkish institutions. The cleric has denied ordering the investigations.

Erdogan maintained his self-declared witch-hunt against Gulen's network in the early part of the year, as he contested presidential elections.

A number of officials linked to Gulen were arrested and charged with belonging to a terrorist group.

While Erdogan's anti-Gulen campaign slowed after he was elected President, it re-appeared last week when he attacked Gulen's supporters and promised to tackle them in their lairs.

It is just over a year since the first corruption probe became public.

While Erdogan has previously said he would ask the United States to extradite Gulen, such a move would require an arrest warrant to be issued.

If a warrant has been issued, it could be a significant moment in a new phase of the bitter struggle between the two men.