A suicide bomber who blew up a bus in Bulgaria last week, killing five Israeli tourists, was backed by an organized group who helped him plan and carry out the attack, Bulgarian Prime Minister Boiko Borisov said on Tuesday.
Borisov said police had not yet identified the bomber, who along with the Bulgarian bus driver also died. More than 30 people were wounded in Wednesday's attack at Burgas airport.
Borisov said the perpetrator had not acted alone.
"These are extremely experienced people who have followed strict conspiracy rules," Borisov told reporters after meeting John Brennan, a U.S. counter-terrorism adviser to President Barack Obama.
"From what we see, they arrived nearly a month beforehand, changed rental cars, and traveled to different cities ... and not more than one of the people we are looking for was captured on either security camera," Borisov said.
He declined to give more details on the plotters.
"There was absolutely no chance of preventing such an act of violence," Borisov insisted, according to The Associated Press. "We could have only detected it by chance or if we had been informed by the services that such activities were under way in Bulgaria."
An explosion rocked a bus carrying Israeli tourists at an airport in Bulgaria, killing at least four people. NBCNews.com's Dara brown reports.
Borisov said that the bomber's DNA and fingerprints had not matched anything held on file by Bulgaria or by partner spy agencies.
He suggested that the attacker, whose bomb was concealed in his backpack, may have entered Bulgaria on a plane from the European Union's "Schengen" passport-free travel zone.
"We do not know his identity, but it is known when he has arrived, the presumed flight, where he came from. It could turn out that he entered Bulgaria from a Schengen member country," Borisov said.
Israel's military chief insisted Tuesday that Iran and Hezbollah were involved and vowed that Israel would respond to the attack.
"We will have to find a way to respond to this attack, and not just a one-off," Lt. Gen. Benny Gantz was quoted as telling the Israeli parliament's foreign affairs and defense committee. "We will know how to do it judiciously. Ultimately, the response will come."
His comments were relayed by a meeting participant who discussed contents from the closed session on condition of anonymity. Iran has denied the accusations.
Borisov said that Bulgaria -- a member of both the European Union and NATO -- would not say who it thought was responsible for the attack until the investigation was complete.
Brennan said the U.S. has been working with Bulgaria on the investigation.
"Bulgaria will continue to have the full support of the United States in the weeks and months ahead," Brennan said.
Reuters and The Associated Press contributed to this report.
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