Tsvetan Tsvetanov said the suspected bomber appeared on security camera tape for nearly an hour before the Wednesday attack, which gutted the bus at the airport in the quiet Black Sea resort city of Burgas, some 400 kilometers (250 miles) east of the capital, Sofia.
The death toll had risen to eight, including the suspected bomber, as the Bulgarian driver of the bus died in the hospital, Tsvetanov said. Six of the victims are Israeli citizens, while the nationality of the suspected bomber remains unknown.
The Israelis had just arrived on a charter flight from Tel Aviv carrying 154 people, including eight children.
No group had immediately claimed responsibility, but suspicion fell upon Iran and its Lebanese proxy, the Hezbollah guerrilla group. Iran's state TV rejected accusations of Tehran's involvement.
A commentary Thursday on the TV website calls claims by Israeli Prime Minister Benjamin Netanyahu and others "ridiculous" and "sensational."
The bombing was the latest in a series of attacks attributed to Iran that have targeted Israelis and Jews overseas and threatened to escalate a shadow war between the two arch-enemies.
"All signs point to Iran," Netanyahu said Wednesday. "Just in the past few months, we have seen attempts by Iran to harm Israelis in Thailand, India, Georgia, Kenya, Cyprus and more. This is an Iranian terror attack that is spreading across the world. Israel will react forcefully to Iran's terror."
The Israeli leader gave no evidence to back his charges.
An Israeli military plane was preparing to fly back 30 wounded Israelis who had been hospitalized in Burgas. Israeli Foreign Ministry spokesman Paul Hirschson said all of the wounded had left the hospital for the airport, some of them on stretchers.
Israeli doctors were to examine two more seriously wounded Israelis to see if their conditions allowed them to fly back to Israel, too, military Dr. David Dagan told Army Radio.
A Bulgarian government plane will fly some 100 Israelis who were not wounded and wanted to cut short their vacation back to Israel.
Black smoke billowed into the sky from the stricken bus after the bomb exploded. Young Israelis said they were just boarding when the blast ripped through the white vehicle in the airport parking lot.
"We were at the entrance of the bus and in a few seconds we heard a huge boom," said Gal Malka, an Israeli teenager who was slightly wounded.
The resort town has become a popular travel destination in recent years for Israelis, particularly for recent high school graduates before they are drafted for mandatory military service.
Despite repeated alerts and concerns of an Iranian-backed attack in recent months, Israel said it had no advance intelligence on a pending attack in Bulgaria.
Late Wednesday, Israel announced it was dispatching a military medical and relief team to Bulgaria, a country of 7.3 million bordering Greece and Turkey.