SOFIA, July 24 (Xinhua) -- It is still unclear who planned the bomb attack that killed five Israeli tourists and a Bulgarian bus driver last week in Burgas, Bulgarian Prime Minister Boyko Borissov said on Tuesday.
"At present, we have no evidence for the offender: what is his nationality, and from where he has come," Borissov told a joint press conference after talks with John Brennan, Assistant to U.S. President Barack Obama for Homeland Security and Counterterrorism.
"DNA profile and fingerprints did not reveal a registered person in the databases of the services in the world," Borissov said.
He added it was "just a matter of time," until more data became available as the investigation progresses.
Brennan said that within hours of the bombing, U.S. and Bulgarian officials were working side-by-side to collect and preserve evidence at the site of the attack.
"It is this ongoing cooperation that allowed us to support the Bulgarian authorities so quickly after last week's heinous attack," he said, adding that the United States would continue to help Bulgaria in any way they can.
Brennan presented to Borissov a letter from President Obama which said the attack on July 18 was "a terrible reminder of the danger we face and the challenges that we must confront as allies."