ISIS flag
A rebel fighter takes away a flag that belonged to Islamic State militants in Akhtarin village, after rebel fighters advanced in the area, in northern Aleppo Governorate, Syria, Oct. 7, 2016. REUTERS/Khalil Ashawi

The fatal shooting of Andrei Karlov, Russia's ambassador to Turkey, by off-duty Turkish police officer Mevlut Mert Altintas has once again highlighted concerns over law enforcement officials being radicalized by terrorist groups such as al Qaeda and the Islamic State group. Altintas is reported to have shouted an Arabic phrase commonly used in propaganda materials of ISIS whose books were found at his house.

While investigators are looking whether Altintas had any connections to the terrorist groups, Turkish President Recep Tayyip Erdoğan on Wednesday blamed the attack on the followers of U.S.-based Muslim cleric Fethullah Gülen, a former ally of the president and now a self-exiled scholar.

“He [Altintas] was a member of the FETO terrorist organization. There is no point in hiding this,” Erdoğan said. “From the places he was raised to his connections — that’s what they point at.”

Turkish officials are also probing Altintas’s links to the Gülen Movement, a name referring to the cleric’s supporters. However, Gülen has denied any involvement in Karlov’s shooting and condemned the attack.

In other instances, U.S. authorities have arrested people with law enforcement ties accused of helping ISIS.

This August, Washington D.C. officials arrested Nicholas Young, a veteran police officer with D.C.’s Metro Transit Police Department, for attempting to help the militant group. The 12-year veteran of the transit police force allegedly bought tech items to send to ISIS operatives to help them in their communication. Authorities caught him after undercover agents with the FBI’s Joint Terrorism Task Force were disguised as ISIS operatives. Young had reportedly been under surveillance since 2010.

According to the Department of Justice, Young traveled to Libya in 2011 and tried to visit the troubled country a second time, telling FBI agents “that he had been with rebels attempting to overthrow the Muammar Qaddafi regime.”

Last July, FBI agents arrested Alexander Ciccolo, the estranged son of a reputed Boston police captain, as a possible terrorist suspect. Authorities reportedly recovered weapons loaded with possible bomb-making equipment, including a pressure cooker, several chemicals, an alarm clock, along with “attack planning papers” and “jihad” paperwork. He went by the name Abu Ali al-Amriki and neighbors said he recently converted to Islam, FBI officials told ABC News at the time.

“This is a very bad person arrested before he could do very bad things,” one senior federal official told ABC News about Ciccolo.