The Israel Defense Forces suspended construction on the underground barrier being built around the Gaza Strip on Wednesday out of concern that workers might come under attack by a Palestinian terror group in Gaza whose tunnel was destroyed in an Israeli detonation on Monday, according to a senior IDF officer.

“The [Palestinian] Islamic Jihad will have a hard time holding back,” said the unnamed senior official in the IDF’s Southern Command on Wednesday.

Early Monday afternoon, the Israel Defense Forces bombed an attack tunnel being built by the terror group extending from the Gazan city of Khan Younis into Israeli territory, near Kibbutz Kissufim.

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The officer said the military had been monitoring the tunnel since before it entered Israeli soil and that it opted to wait until it passed beyond Gazan territory before carrying out the strike. He repeated the military’s position that at no point were Israeli civilians at risk from the tunnel.

Nine Palestinian terrorists — including two senior Palestinian Islamic Jihad commanders and members of Hamas’s elite naval unit — were reported killed in the Israeli strike, and another five were still said missing as of Wednesday and are presumed dead. The IDF officer said the military suspects the missing diggers were working in the stretch of the tunnel that is inside Israeli territory.

According to Israeli assessments, most of the Palestinians who died in the attack tunnel were not killed in the initial blast, but died in its aftermath, from smoke inhalation, secondary explosions, or from the subsequent collapses.

The officer said the Hamas naval commandos had tried to use their special breathing apparatus in the rescue efforts, as there was little oxygen in the collapsed tunnel, but it was to no avail.

“This was a serious failure for [Palestinian Islamic Jihad], both in the fact that we found the tunnel and in the way it chose to rescue those trapped [in the rubble,]” the senior official said.

“But they will be making [another] mistake if they choose to attack in retaliation,” he warned.

The officer said that the number of deaths and pressure from Iran, which backs the terror group, were both motivating factors for a possible retaliation by the Palestinian Islamic Jihad.

Over the past few months the Defense Ministry has been constructing an underground barrier that is designed to thwart attack tunnels like the one destroyed on Monday. The tunnels have in the past been used by the Hamas terrorist group to stage attacks on IDF soldiers or kidnap them into the Gaza Strip to use as bargaining chips with Israel.

The destroyed tunnel is believed to have been built after the 2014 Gaza war, known in Israel as Operation Protective Edge. An IDF official on Monday said the military does not believe the tunnel from Khan Younis is the only border-crossing tunnel constructed since the conflict.

However, the military has also said the new underground barrier being constructed has forced Hamas and other terror groups to focus more on defensive underground infrastructure within the Strip, rather than attack tunnels that enter Israel.

The crews building the underground barrier wear protective vests and have security protection, but the head of the IDF Southern Command, Maj. Gen. Eyal Zamir, halted their work on Wednesday, over fears they could come under fire by sharpshooters from the Palestinian Islamic Jihad.

Local Israeli farmers have been warned against getting too close to the separation fence since Monday out of similar concerns, though the other precautionary instructions that were given to the nearby regional councils were rescinded on Tuesday morning.

The military saw Monday’s operation as a “great achievement,” but cautioned that it was not over yet and so celebrations were premature.

“We’re still in the midst of the event. The operation against the tunnel hasn’t ended yet, not according to us and not according to them,” the officer said.

The military official warned the response from the Palestinian Islamic Jihad might come in the next few days.

He speculated that the group — which is close to Hamas, but distinct from it — could launch rockets at Israeli cities, fire an anti-tank missile at an IDF patrol on the border or carry out a terror attack in the West Bank, as its form of retribution for the at least seven, or more likely 12, of its members that were killed in the tunnel demolition.

The Palestinian Islamic Jihad, Hamas and other terror groups in the Gaza Strip convened an emergency meeting on Monday night to discuss possible responses to the Israeli bombing. According to Palestinian media, Islamic Jihad pushed for an immediate response while Hamas cautioned against it.

One of the reasons behind the disagreement over retaliation was that Israel’s strike on the tunnel came at the same time as Hamas, which rules the Gaza Strip, and the Palestinian Authority, which controls the West Bank, were conducting ongoing reconciliation talks.

As a result of those talks, Hamas handed over control of the border crossings to Gaza to the PA on Wednesday morning.

Hamas is also constantly working to improve its relationship with Egypt, which controls Gaza’s southern Rafah crossing.

An attack by any Palestinian terrorist group from the Gaza Strip could threaten both of those efforts, something Hamas would not condone. However, the officer said, while the Palestinian Islamic Jihad is allied with Hamas, it is not beholden to it and could carry out an attack without Hamas permission.

“If Hamas doesn’t restrain [the Palestinian Islamic Jihad], then Hamas will also enter the cycle of violence,” the senior officer said.

The senior military official said that while the military believes an attack may be delayed, “the more time passes from the event, the lower the chances that the Islamic Jihad will retaliate.”

The IDF officer credited a team that was made up of engineers, geologists and intelligence officers, which he referred to as “the brain” and “the laboratory,” with discovering the tunnel.

This summer, IDF officials promised that Israel will ultimately complete the underground barrier, even under threat, due to its importance.

The officer on Wednesday compared the subterranean obstacle, which will include both an underground concrete wall and advanced detection sensors, to a “guillotine” that will cut off the threat of tunnels.

Israel has been mum on the exact nature of the sensors and technologies that are being used to detect tunnels. But the former US ambassador to Israel, Dan Shapiro, wrote on Tuesday that it was likely developed as part of a joint American-Israeli effort that began under former US president Barack Obama.

“President Obama and Congress approved $40 million in 2016 for the program, with expectations of two additional years, totaling $120 million,” Shapiro wrote on Twitter.

17. Pres. Obama and Congress approved $40 million in 2016 for the program, with expectations of two additional years, totaling $120 million. — Dan Shapiro (@DanielBShapiro) October 31, 2017

The IDF officer said the technologies were “getting us closer to the goal of removing the threat of attack tunnels.”