How did they explode and who is responsible?

Thousands of people have been injured in Lebanon, after pagers used by the armed group Hezbollah to communicate dramatically exploded almost simultaneously across the country on Tuesday.

At least nine people were killed in the blasts, with hundreds more pouring into hospitals across the country. Lebanon’s health ministry says at least 2,750 people have been injured, 200 of them seriously.

It is unclear how the attack – which looks to have been highly sophisticated – occurred, though Hezbollah has blamed its adversary Israel. Israeli officials have so far declined to comment.

Here is what we know so far.

The blasts began in southern Beirut and several other areas of Lebanon on Tuesday afternoon at around 15:45 local time (13:45 BST).

Witnesses reported seeing smoke coming from people’s pockets, before seeing small explosions that sounded like fireworks and gunshots.

In one clip, CCTV footage appeared to show an explosion in a man’s trouser pocket as he stood at a shop till.

Explosions continued for around an hour after the initial blasts, the Reuters news agency reported.

Soon after, scores of people began arriving at hospitals across Lebanon, with witnesses reporting scenes of mass confusion.

A source close to Hezbollah told AFP that two of those killed were the sons of two Hezbollah MPs. They also said the 10-year-old daughter of a Hezbollah member was killed.

Hezbollah chief Sayyed Hassan Nasrallah was not hurt in the explosions, Reuters reported quoting a source.

Analysts have been quick to express shock at the scale of Tuesday’s attack – saying Hezbollah prides itself on its security measures.

Some suggested a hack may have caused the pager batteries to overheat, causing the devices to explode. Such an act would be unprecedented.

But many experts say that is unlikely, with footage of the explosions inconsistent with the batteries overheating.

Some analysts say instead that some sort of supply chain attack, which involved the pagers being tampered with during their manufacture or in transit, was more likely.

Supply chain attacks are a growing concern in the cyber security world with many high-profile incidents recently caused by hackers gaining access to products whilst they are in development.

But these attacks are normally contained to software. Hardware supply chain attacks are far rarer as they involve getting hands on to the device.

If this was indeed a supply chain attack it would have involved a huge operation to secretly tamper with the pagers in some way.

A former British Army munitions expert, who asked not to be named, told the BBC the devices could have been packed with between 10 to 20 grams each of military-grade high explosive, hidden inside a fake electronic component.

This, said the expert, would have been armed by a signal, something called an alphanumeric text message.

So far, nobody has claimed responsibility – though Lebanon’s prime minister and Hezbollah have blamed Israel.

Prime Minister Najib Mikati said the explosions represented a “serious violation of Lebanese sovereignty and a crime by all standards”.

In its statement accusing Israel of being behind the attacks, Hezbollah said it held the country “fully responsible for this criminal aggression that also targeted civilians”.

“This treacherous and criminal enemy will certainly receive its just punishment for this sinful aggression, whether it expects it or not,” it added.

Israeli officials have not commented on the allegations, but most analysts agree that it seems likely it is behind the attack.

Prof Simon Mabon, chair in International Relations at Lancaster University, told the BBC: “We know that Israel has a precedent of using technology to track its target” – but he called the scale of this attack “unprecedented”.

Lina Khatib, from the UK-based Chatham House, said the attack suggested that Israel has “deeply” infiltrated Hezbollah’s “communications network”.

Hezbollah relies heavily on pagers for the group’s communications.

Mobile phones have long since been abandoned as simply too vulnerable, as Israel’s assassination of the Hamas bomb-maker Yahya Ayyash demonstrated as long ago as 1996, when his phone exploded in his hand.

But one Hezbollah operative told the AP news agency that the pagers were a new brand that the group had not used before.

Emily Harding, an ex-analyst with the CIA, said the security breach was deeply embarrassing to Hezbollah.

“A breach of this magnitude is not only physically harmful, but will also make them question their entire security apparatus,” she told the BBC.

“I would expect to see them conduct an intensive internal investigation that will distract them from a potential fight with Israel.”

Hezbollah is allied with Israel’s arch-nemesis in the region, Iran. The group is part of Tehran’s Axis of Resistance and has been engaged in a low-level war with Israel for months, frequently exchanging rocket and missile fire across Israel’s northern border. Entire communities have been displaced from both sides.

The blasts came just hours after Israel’s security cabinet made the safe return of residents to the north of the country an official war goal.

Prime Minister Benjamin Netanyahu told a visiting US official that Israel would “do what is necessary to ensure its security”.

Earlier on Monday, Israel’s domestic security agency said it had thwarted a Hezbollah attempt to assassinate a former official.

Despite the ongoing tensions, observers say that until now both sides have aimed to contain hostilities without crossing the line into full-scale war. But there are fears that the situation could spiral out of control, with Hezbollah already threatening to respond to Tuesday’s explosions.

Additional reporting by Frances Mao