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 explosions and hundreds more were taken to hospitals across the country. The Lebanese Health Ministry says at least 2,750 people were injured, 200 of them seriously.

It is unclear how the attack, which was apparently very sophisticated, took place. However, Hezbollah has blamed its enemy Israel. Israeli officials have so far refused to comment on the matter.

Here’s what we know so far.

The explosions began around 3:45 p.m. local time (1:45 p.m. BST) on Tuesday afternoon in southern Beirut and several other areas of Lebanon.

Witnesses reported seeing smoke rising from people’s pockets and then hearing small explosions that sounded like fireworks and gunshots.

In one clip, surveillance camera video appeared to show an explosion in the pocket of a man standing at the checkout counter in a store.

The Reuters news agency reported that further explosions occurred for about an hour after the first detonations.

Shortly afterwards, large numbers of people began arriving at hospitals across Lebanon. Witnesses reported scenes of utter confusion.

Security sources told Reuters that the sons of senior Hezbollah members were among the injured. Iranian Ambassador Mojtaba Amani also suffered a “superficial injury” and was in hospital for observation, the country’s semi-official news agency Fars reported.

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

Analysts immediately expressed dismay at the scale of Tuesday’s attack and said Hezbollah was proud of its security measures.

Some suspect that a hacker attack may have overheated the pager batteries and caused the devices to explode. Such an act would be unprecedented.

However, many experts consider this unlikely, as the recordings of the explosions are not consistent with overheating of the batteries.

Instead, some analysts say some type of supply chain attack, in which the pagers were tampered with during manufacturing or transportation, is more likely.

Supply chain attacks are becoming an increasingly serious problem in the world of cybersecurity, and there have been numerous high-profile incidents recently where hackers gained access to products while they were still in development.

However, these attacks are usually limited to software. Attacks on the hardware supply chain are far less common because they require access to the device.

If this had indeed been a supply chain attack, it would have required a massive operation to somehow secretly tamper with the pagers.

A former British Army ammunition expert who wishes to remain anonymous, told the BBC The devices may have each contained 10 to 20 grams of high-explosive military explosives, hidden in a fake electronic component.

According to the expert, this would have been activated by a signal, a so-called alphanumeric text message.

So far, no one has claimed responsibility for the incident, although senior Lebanese ministers and Hezbollah have blamed Israel.

Lebanese Information Minister Ziad Makary condemned the pager attacks as “Israeli aggression,” according to Lebanese television station Al-Manar.

Makary said he had also contacted the United Nations to “bring the perpetrator to justice,” Al-Manar reported.

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

“This treacherous and criminal enemy will surely receive his just punishment for this sinful aggression, whether he expects it or not,” it continued.

Israeli authorities have not commented on the allegations, but most analysts agree that Israel is likely behind the attack.

Prof Simon Mabon, Chair of International Relations at Lancaster University, told the BBC: “We know there is already a precedent for Israel using technology to locate its targets” – but he described the scale of this attack as “unprecedented”.

Lina Khatib of the British network Chatham House said the attack indicated that Israel had “deeply” infiltrated Hezbollah’s “communications network”.

Hezbollah relies heavily on pagers for communications.

Mobile phones have long since ceased to be used because they are simply too vulnerable, as demonstrated by the assassination of Hamas bomb maker Yahya Ayyash by Israel in 1996.

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

Emily Harding, a former CIA analyst, said the security breach was deeply embarrassing for Hezbollah.

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

“I would expect them to conduct an intensive internal investigation that would distract them from a possible conflict with Israel.”

Hezbollah is allied with Israel’s arch-enemy in the region, Iran. The group is part of Tehran’s axis of resistance and has been engaged in a small-scale war with Israel for months. They have been exchanging rockets and missiles across Israel’s northern border. Whole communities have been displaced on both sides.

The explosions occurred just hours after the Israeli security cabinet made the safe return of the inhabitants to the north of the country the official war aim.

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

On Monday, Israel’s domestic intelligence service announced that it had foiled an assassination attempt by Hezbollah on a former official.

Despite the ongoing tensions, observers say both sides have so far tried to contain hostilities without crossing the line into a full-scale war. But there are fears that the situation could spiral out of control. Hezbollah has already threatened to respond to Tuesday’s explosions.

Additional reporting by Frances Mao