Destroyed buses are seen at Burgas airport, outside the Black Sea city of Burgas, Bulgaria, some 400 kilometers (250 miles) east of the capital, Sofia, Wednesday, July 18, 2012.
A suicide bomber killed eight people on a bus carrying Israeli tourists in Burgas airport, drawing a pledge by Prime Minister Benjamin Netanyahu to "react powerfully" to what he called "Iranian terror".
Sofia officials have not publicly assigned blame for the bombing, nor has there been comment from Iran or Hezbollah.
Netanyahu's assertion, based on Israel's long-running suspicions that Iranian and Hezbollah agents are waging a covert campaign against its interests abroad, prompted speculation in local media that the Netanyahu government might strike now.
Israel has long threatened to resort to military force to curb Tehran's disputed nuclear programme, but Defence Minister Ehud Barak sounded more nuanced on Thursday about a response to the Bulgaria attack.
Speaking on Israel Radio he said the country would "do everything possible in order to find those responsible, and those who dispatched them, and punish them" - language that appeared to suggest covert action against individuals.
Israel may be reluctant to cross Western partners by rushing into a long-range war which would stretch its military capabilities and possibly draw Iranian reprisals against U.S. interests and disruptions of the global oil supply.
A clash with Hezbollah, which the Israeli military says has stockpiled as many as 80,000 rockets in neighbouring Lebanon, carries the risk of igniting Israel's northern border while it watches with concern the turmoil in neighbouring Syria.
Giora Eiland, a retired Israeli army general who served as national security adviser from 2003 to 2006, played down the possibility that the Bulgaria bombing would push Netanyahu into another war.
"I think that any response, whatever it may be, will not be an immediate response," Eiland told Israel Radio separately.
"Any response, whatever it may be, will not be in the form of an air force operation, or strike - certainly not in Iran over this matter, nor in Lebanon."
Barak, who focussed on Hezbollah's alleged role in the Bulgaria bombing, described it as the bloodiest of a series of recent plots against Israelis, including diplomats, abroad.
Iran denied involvement in previous attacks but some analysts believe it is trying to avenge the assassination of scientists from its nuclear programme, which it blamed on Israel and Western allies. Iran says its atomic ambitions are peaceful, denying foreign allegations of secret military designs.
Hezbollah has its own scores to settle with Israel. Two years after their 2006 border war, the Lebanese Shi'ite militia lost its commander, Imad Moughniyeh, to a Damascus car bomb it said was the work of Israeli spies, and vowed revenge.
Netanyahu's national security adviser from 2009 to 2011, Uzi Arad, confirmed in a separate interview that Israel killed Moughniyeh - though the country has never formally claimed responsibility.
Arad described the Bulgaria bombing as part of a "dynamic of escalation" but counselled Israel to invest in better intelligence and security.He said "risk management" was required and that Wednesday's bloodshed may be an "unavoidable price" of internal and international pressure building on Iran and its allies.