From Frog News, Bulgaria:
Some initial reports from eyewitnesses indicated it was a female suicide bomber, and it is easy to see why they thought so.
His backpack is large enough to hold a very powerful bomb.
JPost notes:
The man was filmed walking around the airport for an hour prior to the attack by security cameras on the premises, according to the report. His body sustained the most damage in the blast, leading investigators to believe that he set off a bomb located on his person when he boarded the bus.
An FBI check of the document's found on the man's body, including a passport identifying him as a Michigan resident, determined that they were counterfeit, according to the report.
Bulgaria site Novinite says the death toll has risen to eight, as an Israeli victim succumbed overnight as did the Bulgarian bus driver.
Algemeiner has photos of Zaka members recovering bodies from the scene. (Original photos at the Zaka site, some graphic.)
JPost has interviews with people who were on the bus.
One young woman, Gal Malka, who flew to Bulgaria on a vacation before being drafted into the IDF, told Channel two by phone from the scene, “We got on the bus. There were a lot of people on it... Suddenly someone got on there, and something exploded. We heard a boom. And we actually saw body parts. We tried to escape. The door was closed. But there was a hole in the side, through which me and my friend escaped.”
Malksa said there were bodies all around her, and that many people were screaming. She said she bandaged a man who was suffering heavy bleeding from his head.
After the earth-shattering blast, “I opened my eyes and saw that everything around me was fire,” Malka later told Ynet.
UPDATE: Here's a video of the burning bus, from Bulgarian TV:
UPDATE 2: Here is the fake Michigan driver's license that the bomber had:
(h/t Yoel)
UPDATE 3: He was identified.