The statement by the EU's top diplomat Federica Mogherini comes as Turkish media report the CIA has a recording of a phone call in which Saudi crown prince gave instructions to "silence Jamal Khashoggi as soon as possible".
The EU's top diplomat Federica Mogherini said on Thursday all those "really responsible" for the murder of journalist Jamal Khashoggi have to be held accountable.
After Saudi Arabia accepted responsibility and said that 21 people were in custody, with death penalties sought against five men, attention turned to whether the crown prince would be found culpable.
"Those responsible, really responsible for this terrible murder have to be accountable," Mogherini told a news conference in Ankara with Turkish Foreign Minister Mevlut Cavusoglu.
Calling for a "completely transparent and credible investigation", she said: "For us accountability does not mean revenge."
Khashoggi, a US resident who wrote for Washington Post and had been critical of Crown Prince Mohammed bin Salman, was lured to the Saudi consulate in Istanbul on October 2, killed and according to Turkish prosecutors dismembered.
After lengthy denials, Saudi authorities admitted responsibility and said 21 people had been taken into custody. However, a CIA analysis leaked to the US media went further, reportedly pointing the finger at the crown prince.
According to Turkish news website Hurriyet Daily News, the CIA has a recording of a phone call in which Saudi Crown Prince Mohammed bin Salman gave instructions to "silence Jamal Khashoggi as soon as possible".
It cited a prominent Turkish columnist as saying CIA director Gina Haspel had "signalled" the existence of the recording during a visit to Ankara last month.
Turkey has repeatedly called for those who ordered the murder to be held accountable but has stopped short of directly blaming Prince Mohammed.
"We've always been against for instance any application of death penalty but we expect in line with our principles, values, practices on judicial systems full investigation, transparent and fair to take place," Mogherini said.