ANKARA, July 28 (Reuters) - Turkish authorities are seeking up to 15 years in prison for the licence holder of ByLock, an encrypted messaging app Ankara says was used by the group it blames for an attempted military coup in 2016, state-owned Anadolu news agency said on Wednesday. Ankara has cracked down on the network of Fethullah Gulen, a U.S.-based Muslim cleric, since the coup attempt in which some 250 people were killed. It arrested thousands of people for using or possessing Bylock, while hundreds of thousands were sacked, suspended or arrested for alleged ties to Gulen. Ankara outlawed Bylock in the aftermath of the coup, saying Gulen"s followers had used it to communicate on the night of the putsch when a group of rogue soldiers attempted to overthrow the government. Five years after the failed coup, arrests of suspected Gulen supporters are still routine. Gulen has always denied any involvement. Anadolu said on Wednesday that Bylock"s licence-holder David Keynes, a Turkish national previously known as Alpaslan Demir who took U.S. citizenship, turned himself in on June 9. Prosecutors are seeking up to 15 years in jail for "membership of the FETO armed terrorist organisation", using Ankara"s acronym for Gulen"s network, it said. Turkish authorities had issued an arrest warrant for Keynes in December 2020, Anadolu said, adding that Keynes had asked to take advantage of "remorse laws" under which suspects who cooperate can receive lighter sentences. Citing an indictment, Anadolu said Keynes had provided names of individuals, as well as details about Bylock. Rights groups and Turkey"s Western allies have voiced alarm over the crackdown, saying President Tayyip Erdogan has used the abortive coup as a pretext to quash dissent. The government has said the security measures were necessary due to the gravity of the threat Turkey faces.