How did they explode and who is responsible?

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

At least nine people were killed in the blast, and hundreds more were rushed to hospitals across the country. The Ministry of Health in Lebanon says that at least 2,750 people were injured, 200 of them seriously.

It remains unclear how the attack – which appears to have been highly sophisticated – took place, although Hezbollah blamed its enemy Israel. Israeli officials have so far declined to comment.

Here’s what we know so far.

The blasts started south of 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 another clip, CCTV footage is seen bursting into the trouser pocket of a man standing at a food stall.

The explosion continued for about an hour after the first explosion, Reuters news agency reported.

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

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

Hezbollah chief Sayyed Hassan Nasrallah was not injured in the blast, Reuters reported citing a source.

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

Some have suggested that the hack may have caused the pager’s batteries to overheat, causing the devices to explode. Such an act would never happen.

But many experts say that is impossible, as there are pictures of explosions that do not correspond to the heating of batteries.

Some analysts say instead that some kind of supply chain attack, involving pagers being tampered with during production or transit, is more likely.

Supply chain attacks are a growing problem in the cybersecurity world with several high-profile incidents recently caused by hackers gaining access to products during development.

But these attacks are often contained in software. Attacks on the hardware supply chain are extremely rare as they involve getting hands on the device.

If this was indeed a supply chain attack it would involve a great deal of work to secretly disrupt the pagers somehow.

A former British military weapons expert, who asked not to be named, he told the BBC the devices could pack between 10 and 20 grams of high-grade military explosives each, hidden inside a fake electronic component.

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

So far, no one has claimed responsibility – although Lebanon’s prime minister and Hezbollah have blamed Israel.

Prime Minister Najib Mikati said the explosion represented “a serious violation of Lebanon’s sovereignty and a crime by all standards”.

In its statement accusing Israel of being behind the attack, Hezbollah said the country was “fully responsible for this violence against people”.

“This treacherous and criminal enemy will surely receive its due punishment for this sinful violence, whether we expect it or not,” it added.

Israeli officials have not commented on the allegations, but most analysts agree that they appear to be behind the attack.

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

Lina Khatib, of UK-based Chatham House, said the attack suggested Israel had “deeply penetrated” Hezbollah’s “communications network”.

Hezbollah relies heavily on pagers for group communications.

Mobile phones have long been left extremely vulnerable, as Israel’s assassination of Hamas bomb maker Yahya Ayyash demonstrated back in 1996, when his phone exploded in his hand.

But one Hezbollah operative told the AP news agency that the pagers are a new type that the group has never used before.

Emily Harding, a former CIA analyst, said the security breach was a major embarrassment for Hezbollah.

“A breach of this growth is not only physically damaging, but it will also make them question all their security resources,” he told the BBC.

“I would expect to see them do a thorough internal investigation that will distract them from a possible fight with Israel.”

Hezbollah is allied with Israel’s biggest enemy 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, regularly exchanging rockets and missiles along Israel’s northern border. Entire communities have been removed from both sides.

The explosion happened a few hours after the Israeli security cabinet made the safe return of civilians to the north of the country a legitimate war goal.

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

Earlier on Monday, Israel’s security forces said they had foiled an attempt by Hezbollah to assassinate a former official.

Despite the tension, observers say that so far both sides have aimed to limit the fighting without crossing the border into full-scale fighting. But there are fears that the situation could spiral out of control, as Hezbollah has already threatened to respond to Tuesday’s blast.

Additional reporting by Frances Mao