Hundreds of pagers exploded in Lebanon and Syria in a deadly attack. Here’s what we know.

NEW YORK — In what seems like an advanced, remote attackPagers used by hundreds of Hezbollah members exploded almost simultaneously in Lebanon and Syria on Tuesday, in which at least nine people died — including a young girl — and thousands of others were injured.

The Iranian-backed militant group blamed Israel for the deadly explosions, which targeted an unusually wide range of people and showed signs of a long-planned operation. How the attack was carried out is largely uncertain, and investigators did not immediately say how the beepers were detonated. The Israeli military declined to comment.

This is what we know so far.

Hezbollah leader Hassan Nasrallah previously warned the group’s members not to carry mobile phones, as they could be used by Israel to track the group’s movements for targeted attacks. As a result, the organization uses pagers to communicate.

A Hezbollah official told The Associated Press that the detonated devices were a new brand that the group had not used before. The official, who spoke on condition of anonymity because he was not authorized to speak to the press, did not name the brand or supplier.

With little revealed by investigators so far, multiple theories emerged Tuesday about how the attack might have been carried out. Several experts who spoke to The Associated Press suggested the explosions were likely the result of a supply chain disruption.

Very small explosives may have been built into the pagers before they were delivered to Hezbollah, and then all remotely activated at once, possibly with a radio signal, said Carlos Perez, director of security intelligence at TrustedSec. At the time of the attack, “the battery was probably half explosive and half real battery.”

After security camera footage emerged on social media on Tuesday purportedly showing one of the beepers exploding on a man’s hip in a Lebanese market, two munitions experts also said the explosion appeared to be the result of a small explosive device.

“If you look at the video, the size of the detonation is comparable to an electric detonator alone or one containing an extremely small explosive charge,” said Sean Moorhouse, a former British Army officer and expert in explosive ordnance disposal. He estimated that the amount of explosive needed could have been as little as 1.5 grams to 2 grams – about the size of an eraser on the end of a pencil.

Moorhouse explains that the small explosives and the method of detonating them would have had to have been built into the pagers before they were delivered, suggesting the likely involvement of a state actor. He adds that Israel’s foreign intelligence agency, the Mossad, is the most likely suspect with the means to carry out such an attack.

NR Jenzen-Jones, a military weapons expert who is director of Australia-based Armament Research Services, agreed that the scale and sophistication of the attack “almost certainly point to a state actor,” and that Israel has been accused of carrying out such operations in the past. Last year, AP reported that Iran accused Israel of trying sabotaging its ballistic missile program by using defective foreign components that could explodewhere the weapons were damaged or destroyed before they could be used.

Another possibility is that malware was inserted into the pagers’ operating system, causing the device’s batteries to overload and catch fire at some point.

According to a Hezbollah official and Lebanese security officials, the beepers first became hot and exploded in the pockets or hands of the porters on Tuesday afternoon.

These pagers run on lithium-ion batteries, the Hezbollah official said. He claimed the devices exploded as a result of an attack by an Israeli “security operation,” without elaborating.

When overheated, lithium-ion batteries can smoke, melt, and even catch fire. Rechargeable lithium batteries are used in consumer products ranging from cell phones and laptops to electric cars. Lithium battery fires can reach temperatures of up to 590 C (1,100 F).

Still, Moorhouse and others noted that the images and video footage seen Tuesday looked more like the detonation of a small explosive charge than an overheated battery.

“A lithium-ion battery fire is one thing, but I’ve never seen one explode like that. It looks like a small explosive charge,” said Alex Plitsas, a weapons expert at the Atlantic Council.

Another possibility is an electronic pulse “sent from far away that burned the devices and caused their explosion,” said Yehoshua Kalisky, a scientist and senior researcher at the Institute for National Security Studies, a Tel Aviv think tank.

“It was not a random act; it was deliberate and known,” Kalisky added.