A Palestinian Islamic Jihad terrorist was killed on Monday after a tunnel collapsed under Khan Younis, the Gaza Strip’s second-most populous city. Two other Islamic Jihad operatives were wounded, with two more unaccounted for.

The terrorist organization identified the deceased operative as 28-year-old Salah Mohammed al-Astal. “One of our fighters in the Khan Younis Brigade rose to heaven as a martyr Monday night while carrying out preparatory work,” the group said in a statement. “We emphasize that the blood of the martyrs will remain an illuminating lamp for our fighters on the path of might and dignity.”

Another Islamic Jihad operative was killed in a tunnel collapse on July 10 in northern Gaza. Those are the 13th and 14th to have occurred since the beginning of the year, according to The Times of Israel’s tally.

While Hamas remains the main terrorist organization governing the strip, Islamic Jihad recently gained significant backing from Iran. In May, Iran pledged $70 million in financial aid to Islamic Jihad’s armed wing, the Quds Brigade. Iran also backs Gaza’s new Shiite militant group al-Sabirin, which Israel’s channel 2 described as being modeled after Hezbollah. Al-Sabirin is led by a former Islamic Jihad commander and reportedly receives $10 million annually from Iran.

A Hamas operative who was captured in June after illegally crossing into Israel revealed that the terrorist group’s fighters can travel underground throughout the entirety of Gaza. Two tunnels leading into Israel have been discovered by the IDF since April. The Shin Bet, Israel’s internal security service, says that it has received extensive on the tunnel system from arrested Hamas operatives.