CAIRO — A  suicide bomber was behind the deadly blast that tore through a bus carrying South Korean tourists, Egyptian security officials said.

The officials said the bomber boarded the bus while it waited near the Egypt-Israel border crossing at Taba in the Sinai Peninsula. The Egyptian driver and the South Korean guide had disembarked but were close to the bus when Sunday’s blast took place, according to the officials.

South Korean officials said the bombing killed three South Koreans and an Egyptian driver. The tourists were Korean Christians who had saved for years to visit Biblical sites on their church’s 60th anniversary.

“The preliminary investigation shows some tourists disembarked to get their bags. A man walked to the bus. There was an explosion when he reached the third step,” interior ministry spokesman Hany Abdel Latif told AFP.

The tourists were all members of the same church group from the central South Korean county of Jincheon and were on a 12-day trip through Turkey, Egypt and Israel. They were about to cross into Israel when the attack occurred.

The South Korean ambassador to Egypt, Kim Young-So, told Seoul’s MBN TV station that the bus bombing appeared to be a “suicide bombing by a terrorist.”

An al-Qaeda-linked Islamist group, Ansar Bayt al-Maqdis, took responsibility for the bombing. On its Twitter account, the organization promised to continue to attack Egypt’s economy, tourism, and military.

The group, based in the Sinai Peninsula, has deployed several suicide bombers in attacks on police, as well as in a failed attempt to assassinate interior minister Mohamed Ibrahim in September.

Fifteen survivors from the blast crossed into Israel on Monday and are staying at a hotel in the resort city of Eilat, Israel’s Channel 2 news reported. Other tourists who had been on the bus carrying 33 were still hospitalized in the Sinai.

Egypt refused offers of Israeli help after the blast, transporting at least 12 injured people to hospitals in the Sinai peninsula instead of the closer Yoseftal Hospital in Eilat.

Terrorists have killed dozens of tourists in sporadic attacks over the past several decades, mostly recently in a 2009 bombing that killed a French tourist in Cairo.

Maj. (res.) Aviv Oreg, formerly the head of Al Qaeda and Global Jihad desk at the Israel Defense Force’s military intelligence directorate, said the bombing was likely intended to send a message to Jerusalem and Cairo.

For jihadist organizations in the Sinai Peninsula, this sort of attack is ”very sufficient in order to pinpoint that Israel is their target in their aspirations,” he told The Times of Israel.

Police under now deposed president Hosni Mubarak had all but stamped out Islamist militancy after a spate of Sinai resort bombings between 2004 and 2006.

But the three-year period of lawlessness and unrest after Mubarak’s overthrow in 2011 has allowed the militants to regroup in the restive Sinai peninsula and to branch out to the Nile Delta.

“A continuation in attacks on tourists would mean a shift in strategy by jihadist groups that until now targeted the military and police,” said Issandr El Amrani of the International Crisis Group.

“But that cannot be judged after one attack,” he added.

The head of Egypt’s Chamber of Tourism said the attack could have been aimed at hitting the tourism industry, one of Egypt’s top revenue generators.

“The attack aimed at harming tourism in general,” said Elhami al-Zayat.

Sunday’s bombing came as a court in Cairo began trying Morsi and 35 co-defendants on charges of espionage and collusion with militants to launch attacks in Egypt.

Morsi, Egypt’s first democratically elected and civilian president, was ousted after a year in power amid massive protests demanding his resignation.

The military-installed government has since accused Morsi’s Muslim Brotherhood of masterminding the attacks that have also targeted police headquarters in Cairo.

The Brotherhood, now designated as a terrorist group, publicly renounced violence decades ago and denies involvement in the attacks.

The deadliest attacks have been claimed by the Sinai-based Ansar Beit al-Maqdis group, whose leadership is drawn from militant Bedouin who want an Islamist state in the peninsula.

In recent months, ever since Morsi’s ouster, cooperation between Israeli and Egyptian authorities “has been largely enhanced,” Oreg said, moving from the tactical and operational level to the intelligence realm. The Shin Bet, too, has been forced, over the course of the past two years, to realign itself in order to address the growing threats from the Sinai.