The White House isn't yet backing Israel's assertion that Hezbollah, with the help of Iran, was behind this week's deadly bombing of a tourist bus in Bulgaria.

Defense Minister Ehud Barak on Friday reiterated the Israeli belief that Hezbollah was responsible for the terror attack on Wednesday which killed five Israelis and a Bulgarian bus driver. And a U.S. official speaking on the condition of anonymity also told the Associated Press on Thursday that Iranian-backed Hezbollah was behind the attack.

But asked on Friday about possible Hezbollah involvement, White House Press Secretary Jay Carney was circumspect.

"It is certainly the case that Hezbollah and Iran have been bad actors, as a general matter," Carney said. "But we're not, at this point, in a position to make a statement about responsibility. "

Obama spoke by phone with Bulgarian Prime Minister Boiko Borisov on Friday before he delivered remarks in Fort Myers, Fla., on the shooting rampage early this morning at a movie theater in Aurora, Colo.

"The president reiterated his condemnation of the barbaric attack," according to a White House readout of the call with Borisov. "He offered his support for the ongoing investigation, and for the Bulgarian people in this challenging time. The leaders discussed the strong partnership and excellent counterterrorism cooperation between the United States and Bulgaria, and the president expressed his appreciation for Bulgaria's important contributions as a NATO member to international peace and security."

Iran and Hezbollah have denied involvement in the attack, but Tehran has accused the Israeli spy agency Mossad of killing several Iranian nuclear scientists, leading to speculation that the attack might be latest move in a proxy war between Israel and Iran.

In October, the U.S. accused Iranian officials of involvement in a foiled plot to kill Saudi Arabia's ambassador to the United States.