ISTANBUL (Reuters) - Turkish police arrested 15 senior military officers in an investigation into the network of a U.S.-based cleric who Ankara accuses of orchestrating last year’s attempted coup, state-run Anadolu news agency said on Friday.

It said police were seeking one more officer in the operation, focused on the capital Ankara and spread across nine provinces, adding that 12 of the total 16 suspects were serving officers.

Among the suspects were seven colonels and nine lieutenant colonels from Turkey’s gendarmerie force, which maintains security in rural areas, the Hurriyet news website said.

More than 50,000 people, including security personnel and civil servants, have been jailed pending trial in the aftermath of the failed putsch, which the government blames on Islamic preacher Fethullah Gulen. He has denied involvement.

Some 150,000 people have also been suspended or dismissed in a crackdown which rights groups say has been used as a pretext to muzzle dissent but which the government says has been necessary due to the security threats Turkey faces.

Police operations targeting alleged followers of Gulen have been conducted on a near daily basis since the coup attempt in July 2016, and authorities issued arrest warrants for 44 teachers on Friday, Anadolu said.

The suspects were former teachers at Gulen-linked schools which were previously closed by state decree, Anadolu said. Twenty-eight of the teachers were arrested so far.

Schools linked to Gulen’s network have been established across Asia, Africa and the United States and since the putsch, Turkey has asked countries including Afghanistan and Pakistan to close them down.