ANKARA, Turkey — Several dozen engineers and officials working for what law enforcement authorities described as “critical indigenous programs” have been detained as part of a government probe in Turkey.

A total of 48 people were detained as part of an investigation into the dealings of Fethullah Gülen, a Muslim cleric accused of being the mastermind behind the July 2016 coup attempt. Gülen, once a staunch political ally but now a political nemesis of Turkish President Recep Tayyip Erdogan, lives in self-exile in the United States. Erdogan’s government has vehemently demanded his extradition.

Gülen tops Turkey’s official list of terrorists. The 48 defense industry officials detained on request by a Turkish prosecutor are accused of having links with Gülen.

Turkish officials said on condition of anonymity that the detainees include 25 engineers working for Aselsan, a military electronics specialist and Turkey’s largest defense company. They reportedly work for “critical indigenous programs.”

The detainees also include 17 officials working for Turkish Aerospace Industries; military software specialist Havelsan; the procurement authority SSB; and Tubitak, a state scientific research institute. Six other detainees are identified as “private sector officials with links to the defense industry.”

The probe was launched after evidence was produced by the Turkish intelligence and anti-fraud agency.

A law enforcement official said the detainees were found to be working on several critical programs including avionics; gun systems development; an active protection system for the Altay, Turkey’s first indigenous main battle tank in the making; a radar development program for the TF-X, Turkey’s first indigenous fighter jet program; electronic warfare suites; and electro-optical systems.

In March 2017 Turkish prosecutors launched a probe into scores of personnel at Aselsan. An Ankara prosecutor’s office said at the time it had launched legal proceedings against 84 Aselsan employees on charges of relation to a terrorist organization allegedly connected to Gülen.

Turkish officials claim Gülen was the mastermind of a coup attempt on July 15, 2016, in which more than 250 people, including civilians, were killed.

The Turkish government has since asked for Gülen’s extradition from the U.S., but the American government has said independent U.S. courts should decide the matter.