The Istanbul Public Prosecutor's Office is currently investigating the Gülenist Terror Group's (FETÖ) presence in Turkish football.

It is paying special attention to contacts between the criminal group and a group of players, including Emre Belözoğlu, Arda Turan, Okan Buruk and Bülent Korkmaz. All four are current or former members of the Turkish national football team. Barcelona's Turan, who is on loan to Istanbul's Başakşehir, has recently made headlines after getting involved in a brawl with pop singer Berkay Şahin last week.

A fight between the duo broke out at an Istanbul nightclub after Turan allegedly made an inappropriate comment about Şahin's wife. Şahin was injured in the brawl and went to a hospital. Turan, who was carrying a gun, followed him to the hospital and continued the altercation. At one point, the footballer fired a shot.

Turan may face up to 12.5 years in prison for the incident. He was also slapped a TL 2.5 million (over $400,000) fine by his club Başakşehir.

The Turkish language daily, Vatan reported that the public prosecutor believes some top players in Turkey are linked to FETÖ, which used celebrity athletes to attract recruits. The four players' cases are being separately investigated, according to the newspaper.

Meanwhile, Korkmaz, who currently coaches Turkish top flight team Antalyaspor, has denied any links to the terrorist group.

This is the second criminal inquiry into FETÖ presence in Turkish football. It is believed that testimonies by suspects in the previous case may have pointed the authorities to the new direction.

FETÖ, a terrorist group with infiltrators practically everywhere, from law enforcement to the judiciary, moved to seize power on July 15, 2016. A coup attempt by FETÖ infiltrators in the army was foiled and it was followed by a nationwide crackdown on the group.

Footballers were among the thousands detained for links to the group in the aftermath of the coup attempt. A number of sportsmen were investigated for the "football structure" of the terrorist group.

FETÖ, which posed as a charity with religious undertones for decades, sought to expand its clout in Turkish football as well, prosecutors believe.

One of Turkey's Big Three, Fenerbahçe claimed that they were the victims of a plot by prosecutors, judges and police chiefs loyal to the group who prosecuted and jailed its former Chairman Aziz Yıldırım on match-fixing charges in 2012.

Bekir İrtegün, who played for Fenerbahçe for six years before joining Medipol Başakşehir and Zafer Biryol, who played a season for the team, are among the defendants in the FETÖ case.

Biryol and Ömer Çatkıç, who was capped 19 times in the Turkish national team, are the only suspects in jail while the rest were freed pending trial after they confessed ties to the group.

The defendants had admitted attending meetings of FETÖ members, though they claimed they have seen the group only as a religious charity.

Çatkıç is also being accused of using ByLock, an encrypted messaging app used exclusively by FETÖ members, while others are accused of donating to the group.

The investigations are still underway for other suspects in FETÖ's football-related cases. Authorities have already issued arrest warrants for Hakan Şükür and Arif Erdem, two former Galatasaray players.

Şükür lives in the U.S. like FETÖ leader Fetullah Gülen, while Erdem is believed to have fled to Europe.