SOFIA, Bulgaria—Autopsies have provided officials with new information about the suicide bomber who killed five vacationing Israelis and a Bulgarian bus driver in the Black Sea resort town of Burgas, an official said Sunday.

But the identity of the attacker appeared to remain unknown, and Bulgaria's government was continuing to investigate whether he had carried out the attack with the help of an accomplice.

Israel has blamed Wednesday's attack in Bulgaria, and other recent ones on its citizens around the world, on Iran and its proxy groups, including the Lebanese militant group Hezbollah. Iran has denied responsibility.

On Sunday, Israeli Prime Minister Benjamin Netanyahu was asked about the killings in Bulgaria on CBS' "Face the Nation."

"We have unquestionable, full substantiated intelligence that this was done by Hezbollah and backed by Iran," he said. Israel "will do whatever is necessary" to protect innocent Israelis and "there has to be a price exacted" from the terrorists and their backers, said Netanyahu.

Israel has asked other countries in Europe to increase security in the bus parking lots used by Israelis and at airports, and Bulgarian police have responded by stepping up security at their country's airports. From now on, all flights arriving from Israel will not be publicly announced, and Israeli passengers will be kept in a separate and secure area, officials said Sunday.

Bulgaria's government released no new information over the weekend about its ongoing investigation into the bombing in Burgas, which is about 400 kilometers (250 miles) east of the capital, Sofia. The attack occurred shortly after the Israelis boarded a bus outside the airport in Burgas, a popular tourist destination for Israeli tourists.

However, Dr. Galina Mleva, a forensic expert who took part in autopsies on the victims and the attacker, told Bulgarian National TV on Sunday that the bomber did not meet the description of some witnesses, raising the possibility that they had seen an accomplice. Mileva said the bomber "had a white face, light eyes, and very thick brown hair. The facial bones were shattered in the explosion."

On Friday, district prosecutor Kalina Tchapkanova cited witness reports that the suspect had dark eyes.

Evidence gathered during the attack and several days before it have raised questions about whether the bomber was working alone or with an accomplice.

Security camera footage just before the attack showed the suspected bomber wandering in and out of the bus terminal in Burgas, wearing a baseball cap over long hair, a T-shirt and plaid shorts. His bulky backpack was packed with TNT powder. Interior Minister Tsvetan Tsvetanov told reporters last week the bomber was believed to have been about 36 years old and had been in the country between four to seven days.

Bulgarian prosecutors have said a man tried to rent a car in the days before the bombing but was turned down because his ID appeared suspicious. Authorities have examined his fingerprints, his DNA and his fake Michigan driver's license. They also said the man actually had short hair. It was not clear if he was wearing a wig, or if this was a different person from the bomber.

Afrodita Petrova, the owner of the car rental company, told Bulgarian National TV that the suspect had short dark hair when visiting the office. She said he was the same person from the video camera footage and appeared to be wearing a wig.

"He spoke English with an Arab accent," she said.

Tsvetanov has said the investigation has ruled out that the bomber was a Bulgarian citizen, but he did not say how authorities know that. "Now we are focused on finding out the identity of the suicide bomber," he said.

On Monday, Israel is sending a delegation of tourism officials to Bulgaria led by Tourism Minister Stas Misezhnikov to send out a "business as usual" message, not to give in to terrorism and to strengthen Israel's tourism relationship with Bulgaria.

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Amy Teibel contributed from Jerusalem.

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