New details in Hezbollah pager attack point to supply-chain breach by Israel

Many of the affected pagers were from a new shipment that Hezbollah had received recently, part of a nearly yearlong process to swap out older devices. The group said they detonated in rough unison at 3:30 p.m. Survivors reported hearing the pagers beep and display a series of numbers on their screens about five seconds before they detonated.

According to people familiar with Hezbollah’s investigation, the initial assessment is the pagers detonated because an explosive device was planted in the new models. The scenario indicates Israel found its way into Hezbollah’s supply chain to modify the devices that were delivered.

Osher Assor, managing partner at Israeli cybersecurity consulting firm, Auren, said that based on the footage of the explosions it was clear that the explosives were inserted into the devices ahead of time since lithium batteries catch fire before exploding.

Assor said that such an operation would have taken a long period to plan, at least a year, according to his assessment. He noted that the batch of tampered pagers continued to work regularly until the moment they were activated with a specific signal.

“The moment the specific message arrived, the devices were activated," he said. “This is unprecedented—both the size and the scale—we haven’t seen something like that before."

Hezbollah operatives said the group was still reeling from the attack Wednesday, trying to comprehend the extent of the breach and how Israel had managed it. The group threatened again to retaliate for the attack.

Hezbollah ordered more than 5,000 new pagers in the past year, which were distributed to its members as replacements for older models, according to people familiar with the matter. Hezbollah now believes most of them were sabotaged, which resulted in the explosions.

The new pagers hadn’t reached all of Hezbollah’s ranks, and those carrying older devices were spared. Some of those carrying the new devices were spared as well when they didn’t go off because batteries were dead or they hadn’t been turned on, people familiar with the matter said.

When the devices did explode, those who immediately checked their pagers suffered injuries to their hands and eyes, while those who delayed suffered leg and waist injuries. Ultimately the attack killed at least 12 people and injured roughly 2,800.

The blasts were spread out across Lebanon and in areas where Hezbollah has a heavy presence, causing chaos as the wounded poured into emergency rooms at hospitals. Funerals were organized Wednesday for those who died in the explosions, which included a 9-year-old girl. Hezbollah and the Lebanese government blamed Israel.

The apparent attack by Israel on Tuesday risks escalating the low-simmering conflict between the two sides and frustrating U.S. efforts to seal a cease-fire in Gaza that could also allow a diplomatic solution to end the almost-daily exchanges of fire between the Israeli military and Hezbollah.

Hezbollah said Wednesday it would continue to support Hamas in Gaza by firing on the Israeli military.

The firing between Israel and Hezbollah has displaced tens of thousands on both sides of the Israeli-Lebanese border. The Israeli government this week said one of its new war aims in Gaza was to return Israelis to their homes in the north of the country.

Former Israeli military officials said Israel’s attack was likely aimed at forcing Hezbollah to stop the cross-border firing that has displaced Israelis and was designed to show the Lebanese group that there is a price for its support for Hamas.

“The purpose of such an operation was not to escalate, it was to reach a settlement that will allow people to go back to their houses," said Yossi Kuperwasser, a former head of research for Israeli military intelligence. “This act is meant to convince Hezbollah to accept an arrangement that makes that possible."

The attack in Lebanon came hours after Israel’s internal security service, Shin Bet, said it had thwarted a bombing attack by Hezbollah targeting a former senior official in Israel’s security establishment. The agency said the attack was expected to be carried out in the coming days. Hezbollah hasn’t commented on the allegation.

Anat Peled contributed to this article.

Write to Rory Jones at Rory.Jones@wsj.com and Joyu Wang at joyu.wang@wsj.com