News reports coming out of Bulgaria on Thursday identified the bomber as 36-year-old Medhi Ghazali, a Swedish citizen who was detained by U.S. authorities at Guantanamo Bay from 2002-2004.
The blast took place on late Wednesday as the bus was traveling through the Black Sea resort town of Burgas, according to recent reports. The attack killed five Israeli nationals and the bus driver.
"I cannot confirm ... reports that this individual in media reports is, in fact, a former Guantanamo detainee," Little told reporters on Thursday. "I can't rule it out, either. I'm simply saying we don't know at this stage."
National Counterterrorism Adviser John Brennan has reached out to his counterparts in the Bulgarian government and offered any assistance the U.S. may provide in finding out who carried out the attack, White House press secretary Jay Carney said on Thursday.
Shortly after the attack, Israeli Prime Minister Benjamin Netanyahu blamed the bombing on the Iranian-backed terror group Hezbollah.
Netanyahu spoke with President Obama after the bombing Wednesday, but it is unclear whether he reiterated those same concerns to the president during their conversation.
Regarding Iran's potential involvement in the attack, Carney said the connection was unclear at this time and both U.S. and Bulgarian authorities are continuing to investigate the incident.
"We are still working with those two countries and waiting for assessments of responsibility," Carney told reporters on Thursday.
"I would note as a general principle that Hezbollah has backed, and Iran has backed terrorist actions against innocent people in third countries in the past. And we condemn that, as we condemn the heinous assault that took place in Bulgaria and resulted in loss of life," he added.