The Australian extremist suspected of carrying out the New Zealand mosque massacre visited Croatia just over two years ago, police told the Hina news agency on Saturday.
The man believed to be responsible for gunning down 49 people at two mosques in Christchurch on Friday has been identified as 28-year-old Brenton Tarrant.
He was charged in court on Saturday, with investigators now trying to piece together how his extremist views managed to go undetected by the intelligence services.
"The police have information about the movements of this person in December 2016 and January 2017," a spokeswoman for Croatia police said.
Earlier on Saturday, local media said Tarrant had visited Zagreb during that period as well as several other towns on Croatia's Adriatic coast, including Zadar, Sibenik and Dubrovnik.
Croatian investigators are looking into the reasons for his visit, Prime Minister Andrej Plenkovic told HRT national television.
On Friday, Bulgaria also began investigating a recent visit by Tarrant between November 9-15 last year, claiming he wanted "to visit historical sites and study the history of the Balkan country," chief prosecutor Sotir Tsatsarov said.
He had also made a short visit to the Balkans from December 28-30, 2016, travelling by bus across Serbia, Croatia, Montenegro and Bosnia and Herzegovina, they said.
Media reports said the gunman had listened to a Serbian song about convicted war criminal Radovan Karadzic en route to the massacre, and also had the names of two historical Serbian and Montenegro leaders written in Cyrillic on his gun.
Ethnic hatreds fuelled the bloody dissolution of Yugoslavia in the 1990s, sparking wars that left 130,000 dead and displaced millions as borders were redrawn.