At least one of three suspected ISIS suicide bombers who killed 42 people at Istanbul airport is thought to be a foreigner but investigators are struggling to identify them from their limited remains, officials said on Thursday, as they announced the arrest of 13 suspected ISIS militants.

Police detained 13 people, three of them foreigners, in 16 raids across Istanbul in connection with Tuesday night’s gun and bomb attack on Europe’s third-busiest airport, the deadliest in a series of suicide bombings in Turkey this year.

Counter-terrorism teams led by police special forces launched simultaneous raids, two officials told Reuters. Turkish authorities have said they believe ISIS was behind the airport attack.

The pro-government Yeni Safak newspaper said the bombers were from Kyrgyzstan, Uzbekistan and Dagestan in southern Russia, without naming its sources.

“We’re looking into the possibility of foreigners being involved. It is likely that at least one of them is a foreigner, but the investigation is still underway,” one of the officials said, declining to be named or to comment further because details of the investigation have not yet been released.

Three bombers opened fire to create panic outside Ataturk Airport in Tuesday’s attack, before two of them got inside and blew themselves up. The third detonated his explosives at the entrance.

A second Turkish official also said it was probable that at least one bomber was a foreign national but that the three had not yet been identified due to “extensive soft-tissue damage.”

“A medical team is working around the clock to conclude the identification process,” the official said.

Meanwhile, Interior Minister Efkan Ala said the death toll from the gun and suicide bomb spree has risen to 43, with 19 foreigners among the dead and around 230 people injured.

Turkey has suffered a string of deadly attacks in the past year blamed on either ISIS or Kurdish rebels, and the airport attack comes just at the start of the crucial summer tourist season.