Ex-Turkey defender faces matchfixing charges
Akyel, part of the Turkey team which came third at the 2002 World Cup, was among 46 people including footballers from the country's top division detained on Wednesday.
His lawyer Sezgin Guraslan said he was being held on charges of fraud and gang membership.
"My belief in justice is total. It will soon become clear that my client is innocent," Guraslan said.
Authorities released some of them after questioning on Thursday but prosecutors also asked a court to bring charges against a second man, state-run news agency Anatolian said.
Among those released were Arif Erdem, a deputy trainer of top-division Istanbul club Buyuksehir Belediyespor and Taner Gulleri, a striker with the club.
Erdem, also a former national team player, told Turkish broadcaster NTV he was not involved in any wrongdoing but had been held because he was named in an indictment.
The Turkish Football Federation (TFF) confirmed an investigation by Istanbul prosecutors was underway and said in a statement it had filed a complaint about suspected match-fixing and manipulation with prosecutors in Istanbul's Sariyer district court last August.
The TFF added the probe was not related to an investigation into a huge match-fixing ring exposed by prosecutors in the German city of Bochum last November, although information from that investigation had been passed on to Turkish authorities.
The ring in Bochum involved 200 matches, 29 of which were from Turkey's first division.