Six months ago, Hezbollah's secretary-general Hassan Nasrallah called on the group's members and their families in southern Lebanon to forgo their mobile phones.
"Shut it off, bury it, put it in an iron chest and lock it up," he said in a televised speech.
"Do it for the sake of security and to protect the blood and dignity of people."
The Lebanese militant group opted for using pagers instead.
Experts say the assumption was they would be more difficult to track and infiltrate with Israeli spyware.
But thousands of those pagers have been simultaneously exploded in a deadly attack across Lebanon.
At least nine people have been killed and about 2,800 injured, according to local authorities.
What are pagers?
Pagers, also known as beepers, were popular in the 1980s before mobile phone use became widespread.
They are a small, wireless, battery-operated devices that receive short messages through a central operator — either in text or a recording.
They work on radio waves, so there are no Global Positioning System (GPS) tracking capabilities.
Toby Walsh from the UNSW School of Computer Science and Engineering said trying to track a pager would be like trying to track someone listening to a radio broadcast.
"You're not really giving anything about yourself away from them, so they (Hezbollah) thought it would be a more secure way to communicate with each other," he said.
Follow our live blog for the latest updates on the unfolding situation in Lebanon.
How did the attack unfold?
The detonations started around 3:30pm local time in the southern suburbs of Beirut known as Dahiyeh and the eastern Bekaa valley.
They lasted about an hour, with Reuters witnesses and residents of Dahiyeh saying they could still hear explosions at 4:30pm.
According to security sources and footage reviewed by Reuters, some of the detonations took place after the pagers rang, causing the fighters to put their hands on them or bring them up to their faces to check the screens.
Regional news outlets have broadcast CCTV footage showing what appeared to be a small handheld device spontaneously exploding after being placed next to a grocery store cashier where an individual was paying.
At least nine people have been killed and about 2,800 injured, according to local authorities.
That included some non-Hezbollah victims, such as an eight-year-old girl killed in the Bekaa Valley.
A Hezbollah official has called it the group's "biggest security breach" since the Gaza conflict between Israel and its ally Hamas erupted last October.
The war in Gaza broke out after an October 7 attack by Hamas on southern Israel which resulted in more than 1,200 deaths, Israeli authorities say.
And about 250 Israelis and foreigners were taken hostage.
Israel's retaliatory military campaign has so far killed at least more than 40,000 people in Gaza, according to the Gaza health ministry.
The Iranian-backed militant group blamed Israel for the widespread attack in Lebanon and said it would get "its fair punishment".
The Israeli military declined to comment.
What type of pager exploded?
During the last year, there has been heightened conflict between Hezbollah and Israel, on the Israel/Lebanon border, as part of a spillover from the Israel-Gaza war.
A few months ago, Hezbollah officials reportedly instructed the group's members to stop using mobile phones, in fear that Israel could use them to locate and monitor fighters.
The pagers have since been widely distributed across Hezbollah members and allow the group to also communicate in remote areas, including deep in Lebanon's south.
The pagers targeted in the attack were reportedly made by Taiwan-based Gold Apollo, which several sources say were brought into the country in the spring.
A senior Lebanese security source told Reuters that 5,000 of the Gold Apollo beepers had been ordered by the group earlier in the year.
He identified a photograph of the model of the pager, an AP924, which like other pagers wirelessly receives and displays text messages but cannot be used to make telephone calls.
Gold Apollo's founder Hsu Ching-Kuang has since said the company did not make the pagers used in the detonations in Lebanon.
He told reporters that they were made by a company in Europe that had the right to use the Taiwanese firm's brand.
What caused them to explode?
Diplomatic and security sources have speculated that the explosions could have been caused by the devices' batteries detonating, possibly through overheating.
But Paul Christensen, an expert in lithium ion battery safety at Newcastle University, said the damage seemed inconsistent with past cases of such batteries failing.
"What we're talking about is a relatively small battery bursting into flames," he told Reuters.
"We're not talking of a fatal explosion here … My intuition is telling me that it's highly unlikely."
An electronic warfare expert, who spoke to the ABC on condition of anonymity because of security considerations, said the explosions were immediate, not like a lithium battery in "run away" mode, which indicated the pagers were likely modified.
The New York Times reported that Israel hid explosive material within a new batch of the pagers before they were imported to Lebanon, citing American and other officials briefed on the operation.
A senior Lebanese security source told Reuters that up to three grams of explosives were hidden in the new pagers and had gone "undetected" by Hezbollah for months.
Professor Walsh said the reports indicated that the pagers were potentially hacked, and a small amount of explosives were placed inside the casing of the devices.
"They would have had to have physical access to the pager," he said.
A malware would have also been implanted in the the pager network to put it into "self-destruct mode".
When a particular message was received it would have triggered the "adverse code" for the explosives to be detonated.
"They would have chosen the moment when they thought was most opportune to explode all these pages at the same time," Professor Walsh said.
What next?
Experts say the plot appears to have been many months in the making.
"As far as I'm aware, this is a first. We've never seen anything like this, and certainly nothing like this on this scale," Professor Walsh said.
"It's a very indiscriminate attack, because you might put the pager down in your desk, and as we have heard from reports there were various innocent people who were injured by this," he said.
Israeli media reported the IDF was "on alert" after Hezbollah's threat to retaliate, with Defense Minister Yoav Gallant holding a meeting with the military's chief of staff on Tuesday evening, local time.
Director of the Institute of Political Science at Saint Joseph University Sami Nader, who is a former adviser to Lebanon's minister of Labor, said the scope and gravity of the attacks have serious implications for regional stability.
"This is something that has the potential to propel the region into full-blown war. But one question remains," Mr Nader told ABC News Channel.
"Is Hezbollah ready for this full-blown war? Does Hezbollah still have capabilities of such confrontation, and which communication system will they rely on to conduct such an operation?"
ABC/Reuters
