An important task of a U.S. team examining the material seized during two different raids against Islamic State (ISIS) in Syria over the past week is to clarify suspected ties between Turkey and the jihadist group, said Bloomberg opinion columnist Eli Lake on Friday.

Turkey’s relations with ISIS have come under the spotlight after the jihadist group’s leader Abu Bakr al-Baghdadi was killed on Sunday in a U.S. raid in Syria's northwestern province of Idlib, a mere 5 km from the Turkish border. Turkey has 12 observation posts in Idlib, the last major rebel-held enclave in Syria, according to deal made with Russia last year.

The United States confirmed on Monday it had killed Abul Hassan al Muhajir, an ISIS spokesman and potential future leader, in a separate operation in the town of Jarablus, which has been controlled by Turkey since it was taken over by Turkish-backed Syrian rebel groups in 2016.

“Three U.S. national security officials told me that they want to know more about Turkey’s knowledge of Baghdadi’s whereabouts,” Lake said. “One important task for the team now going through the material seized in the Baghdadi raid and another raid that killed organisation’s spokesman, Abul Hassan al-Muhajir, is to map out the relationship between Turkey’s intelligence service and Islamic State.”

The U.S. intelligence officials’ suspicions about ties between Turkey and ISIS are not only based on where Muhajir and Baghdadi were found in Syria, Lake said.

Many ISIS recruits from Europe and Africa used Turkey’s porous southern border to join the organisation in Syria. At the time Turkey was supporting jihadist groups in Syria to oust Syrian President Bashar Assad.

In 2017, the U.S. Treasury noted that the finance minister of ISIS had relocated from Iraq to Turkey, Lake said.

“Turkey has done everything in its power to support the worst actors in the Syrian civil war,” Lake quoted a U.S. official working closely on Syria policy as saying.

Turkey, led by the Islamist-leaning President Recep Tayyip Erdoğan, says that it is more committed than any other country to battling ISIS, rounding up its members in Turkey and fighting the group in Syria. Erdoğan's domestic critics say he is in fact soft on radical Islam and the nation's courts and police are lenient towards suspected ISIS members.

“It was Kurdish-led Syrian Democratic Forces, remember, that cultivated the source that was able to find Baghdadi,” Lake said. “U.S. intelligence analysts may soon find out if the Turks knew where he was all along.”