Bulgaria has asked Australia, Canada, Israel and Lebanon to help investigate a bus bombing that killed six people last year.
"The investigation is entering a new stage: collect direct evidence on the perpetrators of the attack," Interior ministry official Kalin Gueorguiev said on Saturday.
"Requests regarding the criminal investigation have been sent to several countries including Canada, Australia and Israel," he told Nova television.
Interior Minister Tsvetan Tsvetanov said on Friday a request had been sent to Lebanon, where the bomber's two suspected accomplices live.
Five Israeli tourists and their Bulgarian bus driver were killed at Burgas airport on the Black Sea of Bulgaria on 18 July last year.
Mr Tsvetanov said the two suspected accomplices had been identified as Australian and Canadian passport holders who belonged to Hizbollah.
The Standart daily reported on Saturday that Bulgaria has asked Lebanon to arrest and interrogate them.